Low temperature interconnection of nanoparticles

ABSTRACT

A polymeric linking agent enables the manufacture of photovoltaic cells on flexible substrates, including, for example, polymeric substrates. Photovoltaic cells may be fabricated by a relatively simple continuous manufacturing process, for example, a roll-to-roll process, instead of a batch process.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefits of and priority to U.S.patent application Ser. No. 10/057,394 filed on Jan. 25, 2002, to U.S.Provisional Patent Application Serial No. 60/351,691 filed on Jan. 25,2002, to U.S. Provisional Patent Application Serial No. 60/368,832 filedon Mar. 29, 2002, and to U.S. Provisional Patent Application Serial No.60/400,289 filed on Jul. 31, 2002, all of which are owned by theassignee of the instant application and the disclosures of which areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

[0002] The invention relates generally to the field of photovoltaicdevices, and more specifically to chemical structures and methods ofinterconnecting nanoparticles at low temperatures.

BACKGROUND OF THE INVENTION

[0003] Thin film solar cells composed of percolating networks of liquidelectrolyte and dye-coated sintered titanium dioxide were developed byDr. Michael Grätzel and coworkers at the Swiss Federal Institute ofTechnology. These photovoltaic devices fall within a general class ofcells referred to as dye-sensitized solar cells (“DSSCs”).Conventionally, fabrication of DSSCs requires a high temperaturesintering process (>about 400° C.) to achieve sufficientinterconnectivity between the nanoparticles and enhanced adhesionbetween the nanoparticles and a transparent substrate. Although thephotovoltaic cells of Grätzel are fabricated from relatively inexpensiveraw materials, the high temperature sintering technique used to makethese cells limits the cell substrate to rigid transparent materials,such as glass, and consequently limits the manufacturing to batchprocesses and the applications to those tolerant of the rigid structure.Furthermore, the high temperature sintering process increases the costof manufacturing a photovoltaic cell due to the energy required toperform the sintering.

SUMMARY OF THE INVENTION

[0004] The invention in, one embodiment, addresses the deficiencies ofthe prior art by providing a polymeric linking agent that enables thefabrication of thin film solar cells at relatively low temperatures.This enables the manufacture of such cells on flexible substrates,including, for example, those substrates constructed from somewhat heatsensitive polymeric materials. In addition, the invention providesphotovoltaic cells and methods of photovoltaic cell fabrication thatfacilitate their manufacture by a relatively simple, continuousmanufacturing process. For example, a roll-to-roll process can beutilized instead of the batch processes that limited the prior art. Moreparticularly, in one embodiment, the invention provides a method forinterconnecting metal oxide nanoparticles in DSSCs, with reduced or noheating, using a polymeric linking agent. By way of example, metal oxidenanoparticles may be interconnected by contacting the nanoparticles witha suitable polymeric linking agent dispersed in a solvent, such asn-butanol, at about room temperature or at elevated temperatures belowabout 300° C.

[0005] In one aspect, therefore, the invention provides a method ofinterconnecting nanoparticles at low temperature that includes providinga solution having a polymeric linking agent and a solvent and contactinga plurality of metal oxide nanoparticles with the solution at atemperature below about 300° C., where the solution contains thepolymeric linking agent in a concentration sufficient to interconnect atleast a portion of the plurality of metal oxide nanoparticles. Invarious embodiments of the method, the temperature is below about 200°C., below about 100° C., or at about room temperature. In oneembodiment, the polymeric linking agent includes a long chainmacromolecule. The long chain macromolecule may have a backbonestructure substantially similar to the chemical structure of theplurality of metal oxide nanoparticles, and one or more reactive groupschemically bonded to the backbone structure. In another embodiment, theplurality of metal oxide nanoparticles has a chemical structure of theform M_(x)O_(y), where x and y are integers. By way of example, M caninclude Ti, Zr, W, Nb, Ta, Tb, or Sn.

[0006] In one embodiment, the polymeric linking agent is poly(n-butyltitanate). In another embodiment, the solvent of the solution isn-butanol. In various embodiments of the method, the mechanism forinterconnecting at least a portion of the plurality of metal oxidenanoparticles is a mechanical or electrical bridge formed by the one ormore reactive groups binding to the plurality of metal oxidenanoparticles. The plurality of metal oxide nanoparticles may bedisposed, for example, as a thin film on a substrate. In variousembodiments of the method, the metal oxide nanoparticles are disposed onthe substrate by, for example, dipping the substrate into the solutionincluding the polymeric linking agent, spraying the solution includingthe polymeric linking agent onto the substrate, or dispersing thesolution including the polymeric linking agent on the substrate. In oneembodiment, the plurality of metal oxide nanoparticles are dispersedonto the substrate, and then the solution including the polymericlinking agent is deposited onto the substrate. In another embodiment,the method includes the step of contacting the metal oxide nanoparticleswith a modifier solution. In yet another embodiment, the plurality ofmetal oxide nanoparticles includes nanoparticles of titanium oxides,zirconium oxides, zinc oxides, tungsten oxides, niobium oxides,lanthanum oxides, tantalum oxides, tin oxides, terbium oxides, and oneor more combinations thereof.

[0007] In another aspect, the invention provides a polymeric linkingagent solution including (1) a polymeric linking agent of the formula—[O-M(OR)_(i)—]_(m)—; (2) a plurality of metal oxide nanoparticles thathave the formula M_(x)O_(y); and (3) a solvent; where i, m, x, and y areintegers greater than zero. In one embodiment, M is Ti, Zr, Sn, W, Nb,Ta, or Tb. R may be a hydrogen atom, an alkyl, an alkene, an alkyne, anaromatic, or an acyl group. In this embodiment, the solution preferablycontains the polymeric linking agent in a concentration sufficient tointerconnect at least a portion of the plurality of metal oxidenanoparticles at a temperature below about 300° C. In anotherembodiment, the polymeric linking agent solution contains the polymericlinking agent in a concentration sufficient to interconnect at least aportion of the plurality of nanoparticles at a temperature below about100° C. In one embodiment, for example, the polylinker solution is a 1%(by weight) poly(n-butyl titanate) in n-butanol.

[0008] In a further aspect, the invention provides a flexiblephotovoltaic cell including a photosensitized interconnectednanoparticle material and a charge carrier material, both of which aredisposed between first and second flexible and significantly lighttransmitting substrates. The photosensitized interconnected nanoparticlematerial may include nanoparticles linked by a polymeric linking agent.In one embodiment of the photovoltaic cell, the photosensitizedinterconnected nanoparticle material includes particles with an averagesize substantially in the range of about 10 nm to about 40 nm. Inanother embodiment, the photosensitized interconnected nanoparticlematerial is interconnected titanium dioxide nanoparticles. Thephotosensitized interconnected nanoparticle material may be, forexample, zirconium oxides, zinc oxides, tungsten oxides, niobium oxides,lanthanum oxides, tantalum oxides, tin oxides, terbium oxides, or one ormore combinations thereof. The photosensitized interconnectednanoparticle material may include a photosensitizing agent such as axanthine, cyanine, merocyanine, phthalocyanine, and/or pyrrole. Thephotosensitizing agent may include a metal ion, such as divalent ortrivalent metals. The photosensitizing agent may also include at leastone of a ruthenium transition metal complex, an osmium transition metalcomplex, and an iron transition metal complex. In one embodiment of thephotovoltaic cell, the charge carrier material includes a redoxelectrolyte system. In another embodiment, the charge carrier media is apolymeric electrolyte. According to one feature, the charge carriermaterial transmits at least about 60% of incident visible light.

[0009] In one embodiment of the photovoltaic cell, at least one of thefirst and second flexible, significantly light transmitting substratesincludes a transparent substrate (e.g., a polyethylene terephthalatematerial). In another embodiment, the photovoltaic cell includes acatalytic media layer disposed between the first and second flexible,significantly light transmitting substrates. The catalytic media layeris, for example, platinum. In another embodiment, the photovoltaic cellincludes an electrical conductor material disposed on at least one ofthe first and second flexible, significantly light transmittingsubstrates. In another embodiment, the electrical conductor material is,for example, indium tin oxide.

[0010] Other aspects and advantages of the invention will becomeapparent from the following drawings, detailed description, and claims,all of which illustrate the principles of the invention, by way ofexample only.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The foregoing and other objects, features, and advantages of theinvention described above will be more fully understood from thefollowing description of various illustrative embodiments, when readtogether with the accompanying drawings. In the drawings, like referencecharacters generally refer to the same parts throughout the differentviews. The drawings are not necessarily to scale, and emphasis insteadis generally placed upon illustrating the principles of the invention.

[0012]FIG. 1 depicts an exemplary chemical structure of an illustrativeembodiment of a polylinker for nanoparticles of an oxide of metal M, inaccordance with the invention;

[0013]FIG. 2 depicts another exemplary chemical structure of anillustrative embodiment of a polylinker, according to the invention, fornanoparticles of an oxide of metal M;

[0014]FIG. 3A shows an exemplary chemical structure for aninterconnected nanoparticle film with a polylinker, according to anillustrative embodiment of the invention;

[0015]FIG. 3B shows the interconnected nanoparticle film of FIG. 3Aattached to a substrate oxide layer, according to an illustrativeembodiment of the invention;

[0016]FIG. 4 depicts the chemical structure of poly(n-butyl titanate);

[0017]FIG. 5A shows the chemical structure of a titanium dioxidenanoparticle film interconnected with poly(n-butyl titanate), accordingto the invention;

[0018]FIG. 5B shows the interconnected titanium dioxide nanoparticlefilm of FIG. 5A attached to a substrate oxide layer, according to anillustrative embodiment of the invention;

[0019]FIG. 6 is a cross-sectional view of a flexible photovoltaic cell,according to an illustrative embodiment of the invention;

[0020]FIG. 7 depicts an illustrative embodiment of a continuousmanufacturing process that may be used to form the flexible photovoltaiccell shown in FIG. 6;

[0021]FIG. 8 depicts a current-voltage curve for an exemplary solarcell, according to the invention;

[0022]FIG. 9 shows a current-voltage curve for an exemplary solar cell,according to an illustrative embodiment of the invention;

[0023]FIG. 10 shows current-voltage curves for two additional exemplarysolar cells, according to an illustrative embodiment of the invention;

[0024]FIG. 11 depicts the chemical structure of gelation induced by acomplexing reaction of Li⁺ ions with complexable poly(4-vinyl pyridine)compounds, in accordance with an illustrative embodiment of theinvention;

[0025]FIG. 12 shows the chemical structure of a lithium ion complexingwith polyethylene oxide segments, according to another illustrativeembodiment of the invention;

[0026] FIGS. 13A-13C depict chemical structures for exemplaryco-sensitizers, according to illustrative embodiments of the invention;

[0027] FIGS. 14A-14B depict additional exemplary chemical structures ofco-sensitizers, according to illustrative embodiments of the invention;

[0028]FIG. 15 shows a graph of the absorbance of the 455 nm cut-offfilter (GC455) used to characterize photovoltaic cells according to theinvention;

[0029]FIG. 16 shows a graph of the absorbance of diphenylaminobenzoicacid; and

[0030]FIG. 17 depicts an illustrative embodiment of the coating of asemiconductor primer layer coating, according to the invention.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0031] A. Low Temperature Interconnection of Nanoparticles

[0032] As discussed in the summary above, the invention, in oneembodiment, provides a polymeric linking agent (hereinafter a“polylinker”) that enables the fabrication of thin film solar cells atrelatively low “sintering” temperatures (<about 300° C.). Although theterm “sintering” conventionally refers to high temperature (>about 400°C.) processes, as used herein, the term “sintering” is not temperaturespecific, but instead refers generally to the process of interconnectingnanoparticles at any suitable temperature. In one illustrativeembodiment, the invention provides a method for using polylinkers tointerconnect nanoparticles in a thin film solar cells. According toanother illustrative embodiment, the relatively low temperaturesintering process enables the manufacture of such photovoltaic cellsusing flexible polymer substrates. By employing flexible substrates, theinvention also enables a continuous roll-to-roll or web manufacturingprocess to be employed.

[0033]FIGS. 1 and 2 schematically depict chemical structures ofillustrative polylinkers, according to the invention. The particularpolylinker structures depicted are for use with nanoparticles of theformula M_(x)O_(y) where M may be, for example, titanium (Ti), zirconium(Zr), tungsten (W), niobium (Nb), lanthanum (La), tantalum (Ta), terbium(Tb), or tin (Sn) and x and y are integers greater than zero. Accordingto the illustrative embodiment of FIG. 1, the polylinker 100 includes abackbone structure 102, which is similar in structure to the metal oxidenanoparticles, and (OR)_(i) reactive groups, where R may be, forexample, acetate, an alkyl, alkene, alkyne, aromatic, or acyl group; ora hydrogen atom and i is an integer greater than zero. Suitable alkylgroups include, but are not limited to, ethyl, propyl, butyl, and pentylgroups. Suitable alkenes include, but are not limited to, ethene,propene, butene, and pentene. Suitable alkynes include, but are notlimited to, ethyne, propyne, butyne, and pentyne. Suitable aromaticgroup include, but are not limited to, phenyl, benzyl, and phenol.Suitable acyl groups include, but are not limited to, acetyl andbenzoyl. In addition, a halogen including, for example, chlorine,bromine, and iodine may be substituted for the (OR)_(i) reactive groups.

[0034] Referring to FIG. 2, the polylinker 110 has a branched backbonestructure that includes two -M-O-M-O-M-O— backbone structures, whichinclude (OR)_(i) reactive groups and (OR)_(i+1) reactive groups, where Rmay be, for example, one of the atoms, molecules, or compounds listedabove and i is an integer greater than zero. The two backbone structureshave similar structures to the metal oxide nanoparticles. Collectively,the structure depicted in FIG. 2 can be represented by-M(OR)_(i)—O-(M(OR)_(i)—O)_(n)-M(OR)_(i+1), where i and n are integersgreater than zero.

[0035]FIG. 3A depicts schematically the chemical structure 300 resultingfrom interconnecting the M_(x)O_(y) nanoparticles 302 with a polylinker304. In various embodiments, the polylinker 304 has the chemicalstructure of the polylinkers 100 and 110 depicted in FIGS. 1 and 2,respectively. According to the illustrative embodiment, thenanoparticles 302 are interconnected by contacting the nanoparticles 302with a polylinker 304 at or below room temperature or at elevatedtemperatures that are less than about 300° C. Preferably, the polylinker304 is dispersed in a solvent to facilitate contact with thenanoparticles 302. Suitable solvents include, but are not limited to,various alcohols, chlorohydrocarbons (e.g., chloroform), ketones, cyclicand linear chain ether derivatives, and aromatic solvents among others.It is believed that the reaction between surface hydroxyl groups of thenanoparticles 302 with alkoxy groups on the polymer chain of thepolylinker 304 leads to bridging (or linking) the many nanoparticles 302together through highly stable covalent links, and as a result, tointerconnecting the nanoparticles 302. It also is believed that sincethe polylinker 304 is a polymeric material with a chemical structuresimilar to that of the nanoparticles 302, even a few binding (orlinking) sites between the nanoparticles 302 and the polylinker 304leads to a highly interconnected nanoparticle film with a combination ofelectrical and mechanical properties superior to those of a non-sinteredor non-interconnected nanoparticle film. The electrical propertiesinclude, for example, electron and/or hole conducting properties thatfacilitate the transfer of electrons or holes from one nanoparticle toanother through, for example, π-conjugation. The mechanical propertiesinclude, for example, improved flexibility.

[0036] Still referring to FIG. 3A, at low concentrations of thepolylinker 304, a single polylinker 304 polymer can link manynanoparticles 302 forming a cross-linked nanoparticle network. However,by increasing the concentration of the polylinker 304 polymer, morepolylinker 304 molecules may be attached to the surface of thenanoparticles 302 forming polymer-coated nanoparticles 300. Suchpolymer-coated nanoparticles 300 may be processed as thin films due tothe flexibility of the polymer. It is believed that the electronicproperties of the polymer-coated nanoparticles are not affected to asignificant extent due to the similar electronic and structuralproperties between the polylinker polymer and the nanoparticles.

[0037]FIG. 3B depicts the chemical structure 306 of an illustrativeembodiment of the interconnected nanoparticle film 300 from FIG. 3Aformed on a flexible substrate 308 that includes an oxide layer coating310, which is an electrical conductor. In particular, the polylinkersmay be used to facilitate the formation of such nanoparticle films 300on flexible, significantly light transmitting substrates 308. As usedherein, the term “significantly light transmitting substrate” refers toa substrate that transmits at least about 60% of the visible lightincident on the substrate in a wavelength range of operation. Examplesof flexible substrates 308 include polyethylene terephthalates (PETs),polyimides, polyethylene naphthalates (PENs), polymeric hydrocarbons,cellulosics, combinations thereof, and the like. PET and PEN substratesmay be coated with one or more electrical conducting, oxide layercoatings 310 of, for example, indium tin oxide (ITO), a fluorine-dopedtin oxide, tin oxide, zinc oxide, and the like.

[0038] According to one preferred embodiment, by using the illustrativepolylinkers, the methods of the invention interconnect nanoparticles 302at temperatures significantly below 400° C., and preferably below about300° C. Operating in such a temperature range enables the use of theflexible substrates 308, which would otherwise be destructively deformedby conventional high temperature sintering methods. In one illustrativeembodiment, the exemplary structure 306 is formed by interconnecting thenanoparticles 302 using a polylinker 304 on a substrate 308 attemperatures below about 300° C. In another embodiment, thenanoparticles 302 are interconnected using a polylinker 304 attemperatures below about 100° C. In still another embodiment, thenanoparticles 302 are interconnected using a polylinker 304 at aboutroom temperature and room pressure, from about 18 to about 22° C. andabout 760 mm Hg, respectively.

[0039] In embodiments where the nanoparticles are deposited on asubstrate, the reactive groups of the polylinker bind with thesubstrate, substrate coating and/or substrate oxide layers. The reactivegroups may bind to the substrate, substrate coating and/or substrateoxide layers by, for example, covalent, ionic and/or hydrogen bonding.It is believed that reactions between the reactive groups of thepolylinker with oxide layers on the substrate result in connectingnanoparticles to the substrate via the polylinker.

[0040] According to various embodiments of the invention, metal oxidenanoparticles are interconnected by contacting the nanoparticles with asuitable polylinker dispersed in a suitable solvent at or below roomtemperature or at elevated temperatures below about 300° C. Thenanoparticles may be contacted with a polylinker solution in many ways.For example, a nanoparticle film may be formed on a substrate and thendipped into a polylinker solution. A nanoparticle film may be formed ona substrate and the polylinker solution sprayed on the film. Thepolylinker and nanoparticles may be dispersed together in a solution andthe solution deposited on a substrate. To prepare nanoparticledispersions, techniques such as, for example, microfluidizing,attritting, and ball milling may be used. Further, a polylinker solutionmay be deposited on a substrate and a nanoparticle film deposited on thepolylinker.

[0041] In embodiments where the polylinker and nanoparticles aredispersed together in a solution, the resultant polylinker-nanoparticlesolution may be used to form an interconnected nanoparticle film on asubstrate in a single step. In various versions of this embodiment, theviscosity of the polylinker-nanoparticle solution may be selected tofacilitate film deposition using printing techniques such as, forexample, screen-printing and gravure-printing techniques. In embodimentswhere a polylinker solution is deposited on a substrate and ananoparticle film deposited on the polylinker, the concentration of thepolylinker can be adjusted to achieve a desired adhesive thickness. Inaddition, excess solvent may be removed from the deposited polylinkersolution prior to deposition of the nanoparticle film.

[0042] The invention is not limited to interconnection of nanoparticlesof a material of formula M_(x)O_(y). Suitable nanoparticle materialsinclude, but are not limited to, sulfides, selenides, tellurides, andoxides of titanium, zirconium, lanthanum, niobium, tin, tantalum,terbium, and tungsten, and combinations thereof. For example, TiO₂,SrTiO₃, CaTiO₃, ZrO₂, WO₃, La₂O₃, Nb₂O₅, SnO₂, sodium titanate, andpotassium niobate are suitable nanoparticle materials.

[0043] The polylinker may contain more than one type of reactive group.For example, the illustrative embodiments of FIGS. 1-3B depict one typeof reactive group OR. However, the polylinker may include several typesof reactive groups, e.g., OR, OR′, OR″, etc.; where R, R′ and R″ are oneor more of a hydrogen, alkyl, alkene, alkyne, aromatic, or acyl group orwhere one or more of OR, OR′, and OR″ are a halide. For example, thepolylinker may include polymer units of formulas such as,—[O-M(OR)_(i)(OR′)_(j)—]—, and —[O-M(OR)_(i)(OR′)_(j)(OR″)_(k)—]—, wherei, j and k are integers greater than zero.

[0044]FIG. 4 depicts the chemical structure of a representativepolylinker, poly(n-butyl titanate) 400 for use with titanium dioxide(TiO₂) nanoparticles. Suitable solvents for poly(n-butyl titanate) 400include, but are not limited to, various alcohols, chlorohydrocarbons(e.g., chloroform), ketones, cyclic and linear chain ether derivatives,and aromatic solvents among others. Preferably, the solvent isn-butanol. The poly(n-butyl titanate) polylinker 400 contains a branched—Ti—O—Ti—O—Ti—O— backbone structure with butoxy (OBu) reactive groups.

[0045]FIG. 5A depicts the chemical structure of a nanoparticle film 500,which is constructed from titanium dioxide nanoparticles 502interconnected by poly(n-butyl titanate) polylinker molecules 504. It isbelieved that the reaction between surface hydroxyl groups of the TiO₂nanoparticles 502 with butoxy groups 506 (or other alkoxy groups) of thepolylinker 504 leads to the bridging (or linking) of many nanoparticles502 together through highly stable covalent links, and as a result,interconnecting the nanoparticles 502. Furthermore, it is believed thatsince the polylinker 504 is a polymeric material with a chemicalstructure similar to that of TiO₂, even a few binding (or linking) sitesbetween nanoparticles 502 and polylinker 504 will lead to a highlyinterconnected nanoparticle film 500, with electronic and mechanicalproperties superior to those of a non-sintered or non-interconnectednanoparticle film.

[0046]FIG. 5B depicts the chemical structure 508 of the nanoparticlefilm 500 from FIG. 5A formed on a substrate 510, which includes anelectrically-conducting oxide layer coating 512, by applying thepolylinker solution to the substrate 510 and then depositing thenanoparticles 502 on the polylinker 504. In the illustrative exampleusing titanium dioxide nanoparticles 502, a polylinker solutionincluding poly(n-butyl titanate) 504 is dissolved in n-butanol andapplied to the substrate 510. The concentration of the polylinker 504can be adjusted to achieve a desired adhesive thickness for thepolylinker solution. A titanium dioxide nanoparticulate film 500 is thendeposited on the polylinker coated substrate 510. Reaction between thesurface hydroxyl groups of the TiO₂ nanoparticles with reactive butoxygroups 506 (or other alkoxy groups) of poly(n-butyl titanate) 504results in interconnecting the nanoparticles 502, as well as connectingnanoparticles 502 with the oxide layers 512 on the substrate 510.

[0047]FIG. 6 depicts a flexible photovoltaic cell 600, in accordancewith the invention, that includes a photosensitized interconnectednanoparticle material 603 and a charge carrier material 606 disposedbetween a first flexible, significantly light transmitting substrate 609and a second flexible, significantly light transmitting substrate 612.In one embodiment, the flexible photovoltaic cell further includes acatalytic media layer 615 disposed between the first substrate 609 andsecond substrate 612. Preferably, the photovoltaic cell 600 alsoincludes an electrical conductor 618 deposited on one or both of thesubstrates 609 and 612. The methods of nanoparticle interconnectionprovided herein enable construction of the flexible photovoltaic cell600 at temperatures and heating times compatible with such substrates609 and 612.

[0048] The flexible, significantly light transmitting substrates 609 and612 of the photovoltaic cell 600 preferably include polymeric materials.Suitable substrate materials include, but are not limited to, PET,polyimide, PEN, polymeric hydrocarbons, cellulosics, or combinationsthereof. Further, the substrates 609 and 612 may include materials thatfacilitate the fabrication of photovoltaic cells by a continuousmanufacturing process such as, for example, a roll-to-roll or webprocess. The substrate 609 and 612 may be colored or colorless.Preferably, the substrates 609 and 612 are clear and transparent. Thesubstrates 609 and 612 may have one or more substantially planarsurfaces or may be substantially non-planar. For example, a non-planarsubstrate may have a curved or stepped surface (e.g., to form a Fresnellens) or be otherwise patterned.

[0049] According to the illustrative embodiment, an electrical conductor618 is deposited on one or both of the substrates 609 and 612.Preferably, the electrical conductor 618 is a significantly lighttransmitting material such as, for example, ITO, a fluorine-doped tinoxide, tin oxide, zinc oxide, or the like. In one illustrativeembodiment, the electrical conductor 618 is deposited as a layer betweenabout 100 nm and about 500 nm thick. In another illustrative embodiment,the electrical conductor 618 is between about 150 nm and about 300 nmthick. According to a further feature of the illustrative embodiment, awire or lead line may be connected to the electrical conductor 618 toelectrically connect the photovoltaic cell 600 to an external load.

[0050] The photosensitized interconnected nanoparticle material 603 mayinclude one or more types of metal oxide nanoparticles, as described indetail above. In one embodiment, the photosensitized interconnectednanoparticle material 603 includes nanoparticles with an average size ofbetween about 2 nm and about 100 nm. In another embodiment, thephotosensitized nanoparticle material 603 includes nanoparticles with anaverage size of between about 10 nm and about 40 nm. Preferably, thenanoparticles are titanium dioxide particles having an average particlesize of about 20 nm.

[0051] A wide variety of photosensitizing agents may be applied toand/or associated with the nanoparticles to produce the photosensitizedinterconnected nanoparticle material 603. The photosensitizing agentfacilitates conversion of incident visible light into electricity toproduce the desired photovoltaic effect. It is believed that thephotosensitizing agent absorbs incident light resulting in theexcitation of electrons in the photosensitizing agent. The energy of theexcited electrons is then transferred from the excitation levels of thephotosensitizing agent into a conduction band of the interconnectednanoparticles 603. This electron transfer results in an effectiveseparation of charge and the desired photovoltaic effect. Accordingly,the electrons in the conduction band of the interconnected nanoparticlesare made available to drive an external load electrically connected tothe photovoltaic cell. In one illustrative embodiment, thephotosensitizing agent is sorbed (e.g., chemisorbed and/or physisorbed)on the interconnected nanoparticles 603. The photosensitizing agent maybe sorbed on the surfaces of the interconnected nanoparticles 603,throughout the interconnected nanoparticles 603, or both. Thephotosensitizing agent is selected, for example, based on its ability toabsorb photons in a wavelength range of operation, its ability toproduce free electrons (or electron holes) in a conduction band of theinterconnected nanoparticles 603, and its effectiveness in complexingwith or sorbing to the interconnected nanoparticles 603. Suitablephotosensitizing agents may include, for example, dyes that includefunctional groups, such as carboxyl and/or hydroxyl groups, that canchelate to the nanoparticles, e.g., to Ti(IV) sites on a TiO₂ surface.Examples of suitable dyes include, but are not limited to, anthocyanins,porphyrins, phthalocyanines, merocyanines, cyanines, squarates, eosins,and metal-containing dyes such as, for example,cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)(“N3 dye”);tris(isothiocyanato)-ruthenium(II)-2,2′:6′,2″-terpyridine-4,4′,4″-tricarboxylicacid;cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium; cis-bis(isocyanato) (2,2′-bipyridyl-4,4′dicarboxylato) ruthenium (II); andtris(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium (II) dichloride, allof which are available from Solaronix.

[0052] The charge carrier material 606 portion of the photovoltaic cellsmay form a layer in the photovoltaic cell, be interspersed with thematerial that forms the photosensitized interconnected nanoparticlematerial 603, or be a combination of both. The charge carrier material606 may be any material that facilitates the transfer of electricalcharge from a ground potential or a current source to the interconnectednanoparticles 603 (and/or a photosensitizing agent associatedtherewith). A general class of suitable charge carrier materials caninclude, but are not limited to solvent based liquid electrolytes,polyelectrolytes, polymeric electrolytes, solid electrolytes, n-type andp-type transporting materials (e.g., conducting polymers), and gelelectrolytes, which are described in more detail below.

[0053] Other choices for the charge carrier material 606 are possible.For example, the electrolyte composition may include a lithium salt thathas the formula LiX, where X is an iodide, bromide, chloride,perchlorate, thiocyanate, trifluoromethyl sulfonate, orhexafluorophosphate. In one embodiment, the charge carrier material 606includes a redox system. Suitable redox systems may include organicand/or inorganic redox systems. Examples of such systems include, butare not limited to, cerium(III) sulfate/cerium(IV), sodiumbromide/bromine, lithium iodide/iodine, Fe²⁺/Fe³⁺, Co²⁺/Co³⁺, andviologens. Furthermore, an electrolyte solution may have the formulaM_(i)X_(j), where i and j are ≧1. X is an anion, and M is selected fromthe group consisting of Li, Cu, Ba, Zn, Ni, lanthanides, Co, Ca, Al, andMg. Suitable anions include, but are not limited to, chloride,perchlorate, thiocyanate, trifluoromethyl sulfonate, andhexafluorophosphate.

[0054] In some illustrative embodiments the charge carrier material 606includes a polymeric electrolyte. In one version, the polymericelectrolyte includes poly(vinyl imidazolium halide) and lithium iodide.In another version, the polymeric electrolyte includes poly(vinylpyridinium salts). In still another embodiment, the charge carriermaterial 606 includes a solid electrolyte. In one version, the solidelectrolyte includes lithium iodide and pyridinium iodide. In anotherversion, the solid electrolyte includes substituted imidazolium iodide.

[0055] According to some illustrative embodiments, the charge carriermaterial 606 includes various types of polymeric polyelectrolytes. Inone version, the polyelectrolyte includes between about 5% and about100% (e.g., 5-60%, 5-40%, or 5-20%) by weight of a polymer, e.g., anion-conducting polymer, about 5% to about 95%, e.g., about 35-95%,60-95%, or 80-95%, by weight of a plasticizer and about 0.05 M to about10 M of a redox electrolyte, e.g., about 0.05 M to about 10 M, e.g.,0.05-2 M, 0.05-1 M, or 0.05-0.5 M, of organic or inorganic iodides, andabout 0.01 M to about 1 M, e.g., 0.05-5 M, 0.05-2 M, or 0.05-1 M, ofiodine. The ion-conducting polymer may include, for example,polyethylene oxide (PEO), polyacrylonitrile (PAN),polymethylmethacrylate (acrylic) (PMMA), polyethers, and polyphenols.Examples of suitable plasticizers include, but are not limited to, ethylcarbonate, propylene carbonate, mixtures of carbonates, organicphosphates, butyrolactone, and dialkylphthalates.

[0056] Preferably, the flexible photovoltaic cell 600 also includes acatalytic media layer 615 disposed between the substrates 609 and 612.According to the illustrative embodiment, the catalytic media layer 615is in electrical contact with the charge carrier material 606. Thecatalytic media 615 may include, for example, ruthenium, osmium, cobalt,rhodium, iridium, nickel, activated carbon, palladium, platinum, or holetransporting polymers (e.g., poly(3,4-ethylene dioxythiophene andpolyaniline). Preferably, the catalytic media 615 further includestitanium, or some other suitable metal, to facilitate adhesion of thecatalytic media to a substrate and/or substrate coating. Preferably, thetitanium is deposited in regions or a layer about 10 Å thick. In oneembodiment, the catalytic media 615 includes a platinum layer betweenabout 13 Å and about 35 Å thick. In another embodiment, the catalyticmedia 615 includes a platinum layer between about 15 Å and about 50 Åthick. In another embodiment, the catalytic media 615 includes aplatinum layer between about 50 Å and about 800 Å thick. Preferably, thecatalytic media 615 includes a platinum layer about 25 Å thick.

[0057] In another aspect, the invention also provides methods of forminga layer of interconnected metal oxide nanoparticles on a substrate usinga continuous manufacturing process, such as, for example, a roll-to-rollor web process. These methods may be used, for example, to produceDSSCs. The current processes for producing DSSCs in large numbers, forexample using a continuous and cost effective assembly line process, areextremely difficult at best. The difficulties associated with acontinuous assembly process for a DSSC may arise from the cell supportor substrate, which is generally rigid and typically includes thermallyresistant materials such as glass and metal. The primary reason for thisis related to the high temperature sintering process for producing fusednanocrystals (typically about 400-500° C.). Rigid substrate materials,by their very nature, generally do not lend themselves to a continuousprocess for manufacture, but rather to a more expensive batch process.

[0058]FIG. 7 depicts an illustrative embodiment of a continuousmanufacturing process 700 that may be used to form the photovoltaic cellshown in FIG. 6. According to the illustrative embodiment, aninterconnected nanoparticle film is formed on an advancing substratesheet 705, which may be continuously advanced, periodically advanced,and/or irregularly advanced during a manufacturing run using rollers708. In this illustrative embodiment, the electrical conductor material710, which serves as the basis for one electrode of a photovoltaic cell,is deposited on the advancing substrate 705. In various embodiments, theelectrical conductor material 710 may be deposited on a target region ofthe substrate 705 by thermal evaporation or low temperature sputtering.In addition, the electrical conductor material 710 may be deposited, forexample, by vacuum deposition.

[0059] According to the illustrative embodiment shown in FIG. 7, thephotosensitized nanoparticle material 715 is then deposited. Asdescribed above, the photosensitized nanoparticle material 715 may beformed by applying a solution having a polylinker and metal oxidenanoparticles onto the advancing substrate sheet 705. Thepolylinker-nanoparticle solution may be applied by any suitabletechnique including, but not limited to, dip tanks, extrusion coating,spray coating, screen printing, and gravure printing. In otherillustrative embodiments, the polylinker solution and metal oxidenanoparticles are separately applied to the advancing substrate sheet705 to form the photosensitized nanoparticle material 715. In oneillustrative embodiment, the polylinker solution is applied to theadvancing substrate 705 and the metal oxide nanoparticles (preferablydispersed in a solvent) are disposed on the polylinker. In anotherillustrative embodiment, the metal oxide nanoparticles (preferablydispersed in a solvent) are applied to the advancing substrate 705 andthe polylinker solution is applied to the nanoparticles to form thephotosensitized nanoparticle material 715. As described above withregard to FIG. 6, a wide variety of photosensitizing agents may beapplied to and/or associated with the nanoparticles to produce thephotosensitized nanoparticle material 715.

[0060] After deposition of the photosensitized nanomatrix material 715,the substrate sheet 705 may proceed to further processing stationsdepending on the ultimate product desired. According to thisillustrative embodiment, the charge carrier material 720, whichfacilitates the transfer of electrical charge from a ground potential ora current source to the photosensitized nanoparticle material 715, isdeposited. The charge carrier material 720 may be applied by, forexample, spray coating, roller coating, knife coating, or blade coating.The charge carrier media 720 may be prepared by forming a solutionhaving an ion-conducting polymer, a plasticizer, and a mixture ofiodides and iodine. The polymer provides mechanical and/or dimensionalstability; the plasticizer helps the gel/liquid phase transitiontemperature; and the iodides and iodine act as redox electrolytes.

[0061] Still referring to FIG. 7, the catalytic media layer 725, whichfacilitates the transfer of electrons ejected by the photoexcitedmolecules within the photovoltaic cell, is then deposited. Subsequently,a second electrical conductor layer 730 is deposited. The secondelectrical conductor layer 730 serves as the basis for a secondelectrode of the photovoltaic cell. A second, flexible substrate 735 isthen unwound and applied to the advancing sheet 705 to complete thephotovoltaic cell using the continuous manufacturing process 700.

[0062] Further illustrative examples of the invention in the context ofa DSSC including titanium dioxide nanoparticles are provided below. Thefollowing examples are illustrative and not intended to be limiting.Accordingly, it is to be understood that the invention may be applied toa wide range of nanoparticles including, but not limited to, SrTiO₃,CaTiO₃, ZrO₂, WO₃, La₂O₃, Nb₂O₅, sodium titanate, and potassium niobatenanoparticles. In addition, it should be realized that the invention isgenerally applicable to formation of interconnected nanoparticles for awide variety of applications in addition to DSSC, such as, for example,metal oxide and semiconductor coatings.

EXAMPLE 1 Dip-Coating Application of Polylinker

[0063] In this illustrative example, a DSSC was formed as follows. Atitanium dioxide nanoparticle film was coated on a SnO₂:F coated glassslide. The polylinker solution was a 1% (by weight) solution of thepoly(n-butyl titanate) in n-butanol. In this embodiment, theconcentration of the polylinker in the solvent was preferably less than5% by weight. To interconnect the particles, the nanoparticle filmcoated slide was dipped in the polylinker solution for 15 minutes andthen heated at 150° C. for 30 minutes. The polylinker treated TiO₂ filmwas then photosensitized with a 3×10⁻⁴ N3 dye solution for 1 hour. Thepolylinker treated TiO₂ film coated slide was then fabricated into a 0.6cm² photovoltaic cell by sandwiching a triiodide based liquid redoxelectrolyte between the TiO₂ film coated slide a platinum coated SnO₂:Fglass slide using 2 mil SURLYN 1702 hot melt adhesive available fromDuPont. The platinum coating was approximately 60 nm thick. The cellexhibited a solar conversion efficiency of as high as 3.33% at AM 1.5solar simulator conditions (i.e., irradiation with light having anintensity of 1000 W/m²). The completed solar cells exhibited an averagesolar conversion efficiency (“η”) of 3.02%; an average open circuitvoltage (“V_(oc)”) of 0.66 V; an average short circuit current(“I_(sc)”) of 8.71 mA/cm², and an average fill factor of 0.49 (0.48 to0.52). FIG. 8 depicts a graph 800 that shows the current-voltage curve802 for the dip-coated photovoltaic cell.

EXAMPLE 2 Polylinker-Nanoparticle Solution Application

[0064] In this illustrative example, a 5.0 mL suspension of titaniumdioxide (P25, which is a titania that includes approximately 80% anataseand 20% rutile crystalline TiO₂ nanoparticles and which is availablefrom Degussa-Huls) in n-butanol was added to 0.25 g of poly(n-butyltitanate) in 1 mL of n-butanol. In this embodiment, the concentration ofthe polylinker in the polylinker-nanoparticle solution was preferablyless than about 50% by weight. The viscosity of the suspension changedfrom milk-like to toothpaste-like with no apparent particle separation.The paste was spread on a patterned SnO₂:F coated glass slide using aGardner knife with a 60 μm thick tape determining the thickness of wetfilm thickness. The coatings were dried at room temperature forming thefilms. The air-dried films were subsequently heat treated at 150° C. for30 minutes to remove solvent, and sensitized overnight with a 3×10⁻⁴ MN3 dye solution in ethanol. The sensitized photoelectrodes were cut intodesired sizes and sandwiched between a platinum (60 nm thick) coatedSnO₂:F coated glass slide and a tri-iodide based liquid electrolyte. Thecompleted solar cells exhibited an average η of 2.9% (2.57% to 3.38%)for six cells at AM 1.5 conditions. The average V_(oc) was 0.68 V (0.66to 0.71 V); the average ISC was 8.55 mA/cm² (7.45 to 10.4 mA/cm²); andthe average fill factor was 0.49 (0.48 to 0.52). FIG. 9 depicts a graph900 showing the current-voltage curve 902 for the photovoltaic cellformed from the polylinker-nanoparticle solution.

EXAMPLE 3 DSSC Cells Formed Without Polylinker

[0065] In this illustrative example, an aqueous titanium dioxidesuspension (P25) containing about 37.5% solid content was prepared usinga microfluidizer and was spin coated on a fluorinated SnO₂ conductingelectrode (15 Ω/cm²) that was itself coated onto a coated glass slide.The titanium dioxide coated slides were air dried for about 15 minutesand heat treated at 150° C. for 15 minutes. The slides were removed fromthe oven, cooled to about 80° C., and dipped into 3×10⁻⁴ M N3 dyesolution in ethanol for about 1 hour. The sensitized titanium dioxidephotoelectrodes were removed from dye solution rinsed with ethanol anddried over a slide warmer at 40° C. The sensitized photoelectrodes werecut into small pieces (0.7 cm×0.5-1 cm active area) and sandwichedbetween platinum coated SnO₂:F-transparent conducting glass slides. Aliquid electrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butylpyridine in 3-methoxybutyronitrile was applied between thephotoelectrode and platinized conducting electrode through capillaryaction. Thus constructed photocells exhibited an average solarconversion efficiency of about 3.83% at AM 1.5 conditions. The η at AM1.5 conditions and the photovoltaic characteristics I_(sc), V_(oc),voltage at maximum power output (“V_(m)”), and current at maximum poweroutput (“I_(m)”) of these cells are listed in Table 1 under column A.FIG. 10 depicts a graph 1000 showing the current-voltage curve 1002 forthe photovoltaic cell formed without the polylinker. TABLE 1 B C D E A0.1% 0.4% 1% 2% Untreated polymer soln. polymer soln. polymer soln.polymer soln. η (%) Avg = 3.83  Avg. = 4.30  Avg = 4.55 Avg = 4.15 Avg =4.15 (3.37-4.15) (4.15-4.55)  (4.4-4.82) (3.48-4.46)  (3.7-4.58) I_(sc)Avg = 10.08 Avg = 10.96  Avg = 10.60  Avg = 11.00  Avg = 11.24 (mA/cm²) (8.88-10.86) (10.44-11.5)   (9.79-11.12)  (10.7-11.28) (10.82-11.51)V_(oc) (V) Avg = 0.65  Avg = 0.66  Avg = 0.71 Avg = 0.7  Avg = 0.69(0.65-0.66) (0.6-0.7) (0.69-0.74) (0.69-0.71) (0.68-0.71) V_(m) (V) Avg= 0.454 Avg = 0.46  Avg = 0.50 Avg = 0.45 Avg = 0.44 (0.43-0.49) (0.43-0.477) (0.47-0.53)  (0.4-0.47) (0.42-0.46) I_(m) Avg = 8.4  Avg =9.36  Avg = 9.08 Avg = 9.14 Avg = 9.28 (mA/cm²)  (7.5-8.96) (8.75-9.71)(8.31-9.57) (8.70-9.55) (8.66-9.97)

EXAMPLE 4 DSSC Cells Formed with Various Concentrations of PolylinkerSolution

[0066] In this illustrative example, a P25 suspension containing about37.5% solid content was prepared using a microfluidizer and was spincoated on fluorinated SnO₂ conducting electrode (15 Ω/cm²) coated glassslide. The titanium dioxide coated slides were air dried for about 15minutes and heat treated at 150° C. for 15 minutes. The titanium dioxidecoated conducting glass slide were dipped into a polylinker solutionincluding poly(n-butyl titanate) in n-butanol for 5 minutes in order tocarry out interconnection (polylinking) of nanoparticles. The polylinkersolutions used were 0.1 wt % poly(n-butyl titanate), 0.4 wt %poly(n-butyl titanate), 1 wt % poly(n-butyl titanate), and 2 wt %poly(n-butyl titanate). After 5 minutes, the slides were removed fromthe polylinker solution, air dried for about 15 minutes and heat treatedin an oven at 150° C. for 15 minutes to remove solvent. The slides wereremoved from the oven, cooled to about 80° C., and dipped into 3×10⁻⁴ MN3 dye solution in ethanol for about 1 hour. The sensitized titaniumdioxide photoelectrodes were removed from dye solution, rinsed withethanol, and dried over a slide warmer at 40° C. The sensitizedphotoelectrodes were cut into small pieces (0.7 cm×0.5-1 cm active area)and sandwiched between platinum coated SnO₂:F-transparent conductingglass slides. A liquid electrolyte containing 1 M LiI, 0.05 M iodine,and 1 M t-butyl pyridine in 3-methoxybutyronitrile was applied betweenthe photoelectrode and platinized conducting electrode through capillaryaction. The η at AM 1.5 conditions and the photovoltaic characteristicsI_(sc), V_(oc), V_(m), and I_(m) of the constructed cells are listed inTable 1 for the 0.1 wt % solution under column B, for the 0.4 wt %solution under column C, for the 1 wt % solution under column D, and forthe 2 wt % solution under column E. FIG. 10 depicts the current-voltagecurve 1008 for the photovoltaic cell formed with the polylinker.

EXAMPLE 5 Modifier Solutions

[0067] In this illustrative example, titanium dioxide coated transparentconducting oxide coated glass slides were prepared by spin coatingprocess as described in Example 4. The titanium oxide coated conductingglass slides were treated with polylinker solution including a 0.01 Mpoly(n-butyl titanate) solution in n-butanol for 5 minutes tointerconnect the nanoparticles. The slides were air dried for about 5minutes after removing from the polylinker solution. The slides werelater dipped into a modifier solution for about 1 minute. The modifiersolutions used were 1:1 water/ethanol mixture, 1 M solution of t-butylpyridine in 1:1 water/ethanol mixture, 0.05 M HCl solution in 1:1water/ethanol mixture. One of the slides was treated with steam fromhumidifier for 15 seconds. The slides were air dried for 15 minutes andheat-treated at 150° C. for 15 minutes to remove solvent and thensensitized with a 3×10⁻⁴ M N3 dye solution for 1 hour. The sensitizedphotoelectrodes were sandwiched between platinized SnO₂:F coated glassslides and studied for photovoltaic characteristics using a liquidelectrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butyl pyridinein 3-methoxybutyronitrile. Acid seems to help in increasing thephotoconductivity and efficiency of these photocells. The η at AM 1.5conditions and the photovoltaic characteristics of the cells of thisexample are listed in Table 2 as follows: slides not dipped into amodifier solution and not treated with polylinker solution (column A);slides not dipped into a modifier, but treated with polylinker solution(column B); slides were first treated with polylinker solution and thendipped in 1:1 water/ethanol mixture (column C); slides were firsttreated with polylinker solution and then dipped in 1 M solution oft-butyl pyridine in 1:1 water/ethanol mixture (column D); slides werefirst treated with polylinker solution and then dipped in 0.05 M HClsolution in 1:1 water/ethanol mixture (column E); and slides were firsttreated with polylinker solution and then treated with steam fromhumidifier (column F). TABLE 2 Treated Treated Treated Treated with 1 mt- with 0.05 M Steam from with 0.01 M with 1:1 buPy/1:1 HCl/1:1Humidifier Untreated TiBut EtOH/H₂O EtOH/H₂O EtOH/H₂O for 15 sec. η (%)Avg = 3.92 Avg = 4.41 Avg = 4.11 Avg = 4.34 Avg = 4.67 Avg = 4.41(3.75-4.15) (4.12-4.74) (4.06-4.15) (4.27-4.38) (4.61-4.73) (4.38-4.45)V_(oc) (V) Avg = 0.66 Avg = 0.66 Avg = 0.65 Avg = 0.65 Avg = 0.66 Avg =0.66 (0.66-0.67) (0.65-0.66) (0.64-0.65) (0.64-0.66) (0.65-0.66)(0.66-0.67) I_(sc) Avg = 9.97  Avg = 12.57  Avg = 11.85  Avg = 11.85 Avg = 12.51  Avg = 11.63 (mA/cm²)  (9.48-10.56)  (11.7-13.22)(11.21-12.49) (11.21-12.49) (12.15-12.87) (11.25-12.01) V_(m) (V)  Avg =0.468  Avg = 0.434 Avg = 0.44 Avg = 0.45  Avg = 0.457 Avg = 0.45(0.46-0.48)  (0.4-0.457) (0.43-0.45)  (0.44-0.456) (0.453-0.46) (0.44-0.46) I_(m) Avg = 8.36  Avg = 10.08 Avg = 9.27 Avg = 9.52  Avg =10.23 Avg = 9.67 (mA/cm²) (7.85-8.89)  (9.57-10.37) (9.01-9.53)(9.22-9.75) (10.17-10.29) (9.38-9.96)

EXAMPLE 6 Post-Interconnection Heating to 150° C.

[0068] In this illustrative example, a titanium-dioxide-coated,transparent-conducting-oxide-coated glass slide was prepared by a spincoating process as described in Example 4. The slide was dipped into0.01 M poly(n-butyl titanate) in n-butanol for 30 seconds and wasair-dried for 15 minutes. The slide was later heat treated at 150° C.for 10 minutes in an oven. The heat-treated titanium oxide layer wassensitized with N3 dye solution for 1 hour, washed with ethanol, andwarmed on a slide warmer at 40° C. for 10 minutes. The sensitizedphotoelectrodes were cut into 0.7 cm×0.7 cm active area photocells andwere sandwiched between platinized conducting electrodes. A liquidelectrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butyl pyridinein 3-methoxybutyronitrile was applied between the photoelectrode andplatinized conducting electrode through capillary action. The photocellsexhibited an average η of 3.88% (3.83, 3.9 and 3.92), an average V_(oc)of 0.73 V (0.73, 0.74 and 0.73 V), and an average I_(sc) of 9.6 mA/cm²(9.88, 9.65 and 9.26), all at AM 1.5 conditions.

EXAMPLE 7 Post-Interconnection Heating to 70° C.

[0069] In this illustrative example, a titanium-dioxide-coated,transparent-conducting-oxide-coated glass slide was prepared by a spincoating process as described in Example 4. The slide was dipped into0.01 M poly(n-butyl titanate) in n-butanol for 30 seconds and wasair-dried for 15 minutes. The slide was later heat treated at 70° C. for10 minutes in an oven. The heat-treated titanium oxide layer wassensitized with N3 dye solution for 1 hour, washed with ethanol, andwarmed on a slide warmer at 40° C. for 10 minutes. The sensitizedphotoelectrodes were cut into 0.7 cm×0.7 cm active area photocells andwere sandwiched between platinized conducting electrodes. A liquidelectrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butyl pyridinein 3-methoxybutyronitrile was applied between the photoelectrode andplatinized conducting electrode through capillary action. The photocellsexhibited an average η of 3.62% (3.55, 3.73 and 3.58), an average V_(oc)of 0.75 V (0.74, 0.74 and 0.76 V), and average I_(sc) of 7.96 mA/cm²(7.69, 8.22 and 7.97), all at AM 1.5 conditions.

EXAMPLE 8 Formation on a Flexible, Transparent Substrate

[0070] In this illustrative example, a PET substrate about 200 μm thickand about 5 inches by 8 feet square was coated with ITO and loaded ontoa loop coater. An 18.0 mL suspension of titanium dioxide (P25 with 25%solid content) in n-butanol and 0.5 g of poly(n-butyl titanate) in 10 mLof n-butanol were in-line blended and coated onto the ITO coated PETsheet. After deposition, the coating was heated at about 50° C. forabout 1 minute. The interconnected nanoparticle layer was thendye-sensitized by coating with a 3×10-4 M solution of N3 dye in ethanol.

[0071] B. Gel Electrolytes for DSSCs

[0072] According to further illustrative embodiments, the inventionprovides electrolyte compositions that include multi-complexablemolecules (i.e., molecules containing 2 or more ligands capable ofcomplexing) and redox electrolyte solutions, which are gelled usingmetal ions, such as lithium ions. The multi-complexable compounds aretypically organic compounds capable of complexing with a metal ion at aplurality of sites. The electrolyte composition can be a reversibleredox species that may be liquid by itself or solid components dissolvedin a non-redox active solvent, which serves as a solvent for the redoxspecies and does not participate in reduction-oxidation reaction cycle.Examples include common organic solvents and molten salts that do notcontain redox active ions. Examples of redox species include, forexample, iodide/triiodide, Fe²⁺/Fe³⁺, Co²⁺/Co³⁺, and viologens, amongothers. The redox components are dissolved in non-aqueous solvents,which include all molten salts. Iodide based molten salts, e.g.,methylpropylimidazolium iodide, methylbutylimidazolium iodide,methylhexylimidazolium iodide, etc., are themselves redox active and canbe used as redox active liquids by themselves or diluted with non-redoxactive materials like common organic solvents or molten salts that donot undergo oxidation-reduction reaction cycles. Multi-dendate inorganicligands may also be a source of gelling compounds.

[0073]FIG. 11 depicts an illustrative embodiment of an electrolytegelled using metal ions. Lithium ions are shown complexed withpoly(4-vinyl pyridine). The lithium ions and the organic compounds, inthis instance poly(4-vinyl pyridine) molecules capable of complexing ata plurality of sites with the lithium ions, can be used to gel asuitable electrolyte solution. An electrolyte composition prepared inaccordance with the invention may include small amounts of water, molteniodide salts, an organic polymer, and other suitable compound gels uponthe addition of a metal ion such as lithium. Gelled electrolytes may beincorporated into individual flexible photovoltaic cells, traditionalsolar cells, photovoltaic fibers, interconnected photovoltaic modules,and other suitable devices. The dotted lines shown in FIG. 11 representthe type of bonding that occurs in a photovoltaic gel electrolyte whenthe constituent electrolyte solution and organic compounds gel after theintroduction of a suitable metal ion.

[0074] A non-exhaustive list of organic compounds that are capable ofcomplexing with the metal ion at a plurality of sites, and which aresuitable for use in the invention, include various polymers,starburst/dendrimeric molecules, and other molecules containing multiplefunctional groups, e.g., urethanes, esters, ethylene/propyleneoxide/imines segments, pyridines, pyrimidines, N-oxides, imidazoles,oxazoles, triazoles, bipyridines, quinolines, polyamines, polyamides,ureas, β-diketones, and β-hydroxy ketones.

[0075] More generally, the multi-complexable molecules employed invarious embodiments may be polymeric or small organic molecules thatpossess two or more ligand or ligating groups capable of formingcomplexes. Ligating groups are functional groups that contain at leastone donor atom rich in electron density, e.g., oxygen, nitrogen, sulfur,or phosphorous, among others and form monodentate or multidentatecomplexes with an appropriate metal ion. The ligating groups may bepresent in non-polymeric or polymeric material either in a side chain orpart of the backbone, or as part of a dendrimer or starburst molecule.Examples of monodentate ligands include, for example, ethyleneoxy,alkyl-oxy groups, pyridine, and alkyl-imine compounds, among others.Examples of bi- and multidentate ligands include bipyridines,polypyridines, urethane groups, carboxylate groups, and amides.

[0076] According to various embodiments of the invention, dye-sensitizedphotovoltaic cells having a gel electrolyte 1100 including lithium ionsare fabricated at or below room temperature or at elevated temperaturesbelow about 300° C. The temperature may be below about 100° C., andpreferably, the gelling of the electrolyte solution is performed at roomtemperature and at standard pressure. In various illustrativeembodiments, the viscosity of the electrolyte solution may be adjustedto facilitate gel electrolyte deposition using printing techniques suchas, for example, screen-printing and gravure-printing techniques. Thecomplexing of lithium ions with various ligands can be broken at highertemperatures, thereby permitting the gel electrolyte compositions to beeasily processed during DSSC based photovoltaic module fabrication.Other metal ions may also be used to form thermally reversible orirreversible gels. Examples of suitable metal ions include: Li⁺, Cu²⁺,Ba²⁺, Zn²⁺, Ni²⁺, Ln³⁺ (or other lanthanides), Co²⁺, Ca²⁺, Al³⁺, Mg²⁺,and any metal ion that complexes with a ligand.

[0077]FIG. 12 depicts a gel electrolyte 1200 formed by the complexing ofan organic polymer, polyethylene oxide (PEO), by lithium ions. The PEOpolymer segments are shown as being complexed about the lithium ions andcrosslinked with each other. In another embodiment, the metal ioncomplexed with various polymer chains can be incorporated into areversible redox electrolyte species to promote gelation. The gelelectrolyte composition that results from the combination is suitablefor use in various photovoltaic cell embodiments such as photovoltaicfibers, photovoltaic cells, and electrically interconnected photovoltaicmodules.

[0078] Referring back to FIG. 6, the charge carrier material 606 caninclude an electrolyte composition having an organic compound capable ofcomplexing with a metal ion at a plurality of sites; a metal ion such aslithium; and an electrolyte solution. These materials can be combined toproduce a gelled electrolyte composition suitable for use in the chargecarrier material 606 layer. In one embodiment, the charge carriermaterial 606 includes a redox system. Suitable redox systems may includeorganic and/or inorganic redox systems. Examples of such systemsinclude, but are not limited to, cerium(III) sulfate/cerium(IV), sodiumbromide/bromine, lithium iodide/iodine, Fe²⁺/Fe³⁺, Co²⁺/Co³⁺, andviologens.

[0079] Further illustrative examples of the invention in the context ofa DSSC having a gel electrolyte composition are provided below. Thephotoelectrodes used in the following illustrative examples wereprepared according to the following procedure. An aqueous, titaniasuspension (P25, which was prepared using a suspension preparationtechnique with total solid content in the range of 30-37%) was spun caston SnO₂:F coated glass slides (15 Ω/cm²). The typical thickness of thetitanium oxide coatings was around 8 μm. The coated slides were airdried at room temperature and sintered at 450° C. for 30 minutes. Aftercooling the slides to about 80° C., the slides were immersed into 3×10⁻⁴M N3 dye solution in ethanol for 1 hour. The slides were removed andrinsed with ethanol and dried over slide a warmer at 40° C. for about 10minutes. The slides were cut into about 0.7 cm×0.7 cm square active areacells. The prepared gels were applied onto photoelectrodes using a glassrod and were sandwiched between platinum-coated, SnO₂:F coated,conducting glass slides. The cell performance was measured at AM 1.5solar simulator conditions (i.e., irradiation with light having anintensity of 1000 W/m²).

EXAMPLE 9 Effect of Lithium Iodide in Standard Ionic Liquid BasedElectrolyte Composition

[0080] In this illustrative example, the standard, ionic, liquid-basedredox electrolyte composition that was used contained a mixturecontaining 99% (by weight) imidazolium iodide based ionic liquid and 1%water (by weight), combined with 0.25 M iodine and 0.3 Mmethylbenzimidazole. In various experimental trials, electrolytesolutions with at least a 0.10 M iodine concentration exhibit the bestsolar conversion efficiency. In a standard composition,butylmethylimidazolium iodide (MeBuImI) was used as the ionic liquid.Photovoltage decreased with increases in iodine concentration, whilephotoconductivity and conversion efficiency increased at least up to0.25 M iodine concentration. Adding lithium iodide to the standardcomposition enhanced the photovoltaic characteristics V_(oc) and I_(sc)and the η. Therefore, in addition to lithium's use as a gelling agent,it may serve to improve overall photovoltaic efficiency. Table 3summarizes the effect of LiI on photovoltaic characteristics. TABLE 3Stan- Standard + Standard + Standard + Standard + dard 1 wt % LiI 2 wt %LiI 3 wt % LiI 5 wt % LiI η (%) 2.9% 3.57 3.75 3.70 3.93 V_(oc) (V) 0.590.61 0.6 0.6 0.61 I_(sc) (mA/cm²) 10.08 11.4 11.75 11.79 12.62 V_(m) (V)0.39 0.4 0.39 0.4 0.39 Im (mA/cm²) 7.44 9.02 9.64 9.0 10.23

[0081] The fill factor (“FF”) is referenced below and can be calculatedfrom the ratio of the solar conversion efficiency to the product of theopen circuit voltage and the short circuit current, i.e.,FF=η/[V_(oc)*I_(sc)].

EXAMPLE 10 The Effect of Cations on the Enhancement in PhotovoltaicCharacteristics

[0082] In order to ascertain whether the enhancement in photovoltaiccharacteristics was due to the presence of lithium or iodide, controlledexperimental trials using various iodides in conjunction with cationsincluding lithium, potassium, cesium and tetrapropylammonium iodide wereconducted. The iodide concentration was fixed at 376 mols/gram ofstandard electrolyte composition. The standard composition used was amixture containing 99% MeBuImI and 1% water, combined with 0.25 M iodineand 0.3 M methylbenzimidazole. 376 μmols of various iodide salts pergram of standard electrolyte composition were dissolved in theelectrolyte. The complete dissolution of LiI was observed. The othersalts took a long time to dissolve and did not dissolve completely overthe course of the experimental trial. DSSC-based photovoltaic cells werefabricated using prepared electrolytes containing various cations. Table4 shows the effect of the various cations on the photovoltaiccharacteristics. It is apparent from the second column of Table 4 thatLi⁺ ion shows enhanced photovoltaic characteristics compared to thestandard formula, while the other cations do not appear to contribute tothe enhancement of the photovoltaic characteristics. TABLE 4 Stan-Standard + Standard + Standard + Standard + dard LiI NPR₄I KI CsI η (%)3.23 4.39 2.69 3.29 3.23 V_(oc) (V) 0.58 0.65 0.55 0.58 0.6 I_(sc)(mA/cm²) 10.96 12.03 9.8 9.91 10.14 V_(m) (V) 0.36 0.44 0.36 0.4 0.4I_(m) (mA/cm²) 8.96 9.86 7.49 8.25 8.32

EXAMPLE 11 Effect of Ionic Liquid Type

[0083] In one aspect of the invention, MeBuImI-based electrolytecompositions have been found to perform slightly better than MePrImIbased electrolytes. In addition, experimental results demonstrate that a1/1 blend of MeBuImI and MePrImI exhibit better performance thanMeBuImI, as shown in Table 5. TABLE 5 376 μmoles of LiI per 1 gram of376 μmoles of LiI per 1 gram of MeBuImI based standard MeBuImI/MePrImIbased electrolyte composition. standard electrolyte composition. η (%)3.64 3.99 V_(oc) (V) 0.63 0.63 I_(sc) (mA/cm²) 11.05 11.23 V_(m) (V)0.42 0.42 I_(m) (mA/cm²) 8.69 9.57

EXAMPLE 12 Using Li-Induced Gelling in Composition A Instead of aDibromocompound

[0084] In this illustrative example, a Composition A was prepared bydissolving 0.09 M of iodine in a mixed solvent consisting of 99.5% byweight of 1-methyl-3-propyl imidazolium iodide and 0.5% by weight ofwater. Then, 0.2 g of poly(4-vinylpyridine) (“P4VP”), anitrogen-containing compound, was dissolved in 10 g of the Composition AFurther, 0.2 g of 1,6-dibromohexane, an organic bromide, was dissolvedin the resultant Composition A solution, so as to obtain an electrolytecomposition, which was a precursor to a gel electrolyte.

[0085] Gelling occurred quickly when 5 wt % of lithium iodide (376 μmolsof lithium salt per gram of standard electrolyte composition) was usedas the gelling agent in an electrolyte composition containing (i) 2 wt %P4VP and (ii) a mixture containing 99.5% MePrImI and 0.5% water. The geldid not flow when a vial containing the Li-induced gel was tilted upsidedown. One approach using a dibromo compound produced a phase-segregatedelectrolyte with cross-linked regions suspended in a liquid, which flows(even after gelling at 100° C. for 30 minutes). A comparison of thephotovoltaic characteristics of Composition A, with and without LiI, ispresented in the following Tables 6 and 7. The results demonstrate thatfunctional gels suitable for DSSC-based photovoltaic cell fabricationcan be obtained using lithium ions, while also improving thephotovoltaic characteristics. TABLE 6 Composition Composition A MeBuImIbased A with with 2 electrolyte + 2 wt. % dibromohexane wt. % P4VPP4VP + 5 wt. % LiI η (%) 2.6 3.04 3.92 V_(oc) (V) 0.59 0.58 0.65 I_(sc)(mA/cm²) 9.73 10.0 11.45 V_(m) (V) 0.38 0.38 0.42 I_(m) (mA/cm²) 6.828.04 9.27

[0086] TABLE 7 (a) Composition A where MePrImI:water is 99.5:0.5 andwith 2% (b) Same composition as (a), but with 5 wt P4VP and 0.09 MIodine % of LiI Physical Reddish fluid; flows well Non-scattering Gel;does not flow; can be Properties thinned by applying force using a glassrod. Efficiency 2.53% 3.63% V_(oc) 0.55 V 0.62 V I_(sc) 9.82 mA/cm²12.29 mA/cm² V_(m) 0.343 V 0.378 V FF 0.47 0.47

EXAMPLE 13 Effect of Anions of Lithium Salts on the Efficiency andPhotovoltage of DSSCs

[0087] Experiments were performed to study the effect of counter ions onlithium, given lithium's apparent role in enhancing the overallefficiency of DSSCs. 376 μmols of LiI, LiBr, and LiCl were used per gramof the electrolyte composition containing MePrImI, 1% water, 0.25 Miodine and 0.3 M methylbenzimidazole in order to study the photovoltaiccharacteristics of the cells. The photovoltaic characteristics of cellscontaining these electrolytes are presented in Table 8. TABLE 8Electrolyte composition Electrolyte composition with Electrolytecomposition with LiI LiBr with LiCl Efficiency  3.26%  3.64%  3.71%V_(oc)  0.59 V  0.62 V  0.65 V I_(sc) 10.98 mA/cm² 11.96 mA/cm² 11.55mA/cm² V_(m)  0.385 V  0.4 V  0.40 V FF  0.5  0.49  0.49

EXAMPLE 14 Passivation and Improved Efficiency and Photovoltage of DSSCs

[0088] In the field of photovoltaic cells, the term passivation refersto the process of reducing electron transfer to species within theelectrolyte of a solar cell. Passivation typically includes treating ananoparticle layer by immersion in a solution of t-butylpyridine inmethoxypropionitrile or other suitable compound. After the nanomatrixlayer, such as a titania sponge, of a photovoltaic cell has been treatedwith a dye, regions in the nanomatrix layer where the dye has failed toadsorb may exist. A passivation process is typically performed on a DSSCto prevent the reversible electron transfer reaction from terminating asresult of reducing agents existing at the undyed regions. The typicalpassivation process does not appear to be necessary when ionic liquidcompositions containing various lithium salts and/or other alkali metalsalts are used in the DSSCs. A photovoltage greater than 0.65 V wasachieved using a chloride salt of lithium without a passivation process.

[0089] In this illustrative example, a DSSC was passivated by immersingit in a solution containing 10 wt % of t-butylpyridine inmethoxypropionitrile for 15 minutes. After passivation, the DSSC wasdried on a slide warmer maintained at 40° C. for about 10 minutes.Electrolyte compositions containing MePrImI, 1% water, 0.3 Mmethylbenzimidazole, and 0.25 M iodine were gelled using 376 μmoles ofLiI, LiBr, and LiCl per gram of standard electrolyte composition usedduring this study. Adding a t-butylpyridine-based passivation agent tothe electrolyte enhanced the DSSC's photovoltage, but decreased theefficiency of the DSSC by decreasing the photoconductivity. Table 9summarizes the effects of passivation on photovoltaic characteristics ofelectrolytes containing various lithium halides. TABLE 9 ElectrolyteElectrolyte gelled with Electrolyte gelled with gelled with LiI LiBrLiCl Efficiency  3.5%  3.65%  3.85% V_(oc)  0.61 V  0.63 V  0.65 VI_(sc) 10.96 mA/cm² 11.94 mA/cm² 11.75 mA/cm² V_(m)  0.395 V  0.4 V 0.405 V FF  0.52  0.49  0.5

EXAMPLE 15 Lithium 's Role in Gelling the Electrolyte CompositionsContaining Polyvinylpyridine and the Effect of Other Alkali Metal Ionson Gelability

[0090] Lithium cation appears to have a unique effect in gelling ionicliquid composition containing complexable polymers, e.g., P4VP, in assmall an amount as 2 wt %. Other alkali metal ions such as sodium,potassium, and cesium were used to carry out gelling experiments. Alkalimetal salts such as lithium iodide, sodium chloride, potassium iodide,cesium iodide were added to portions of electrolyte compositioncontaining propylmethylimidazolium iodide (MePrImI), 1% water, 0.25 Miodine, and 0.3 M methylbenzimidazole. Only compositions containinglithium iodide gelled under the experimental conditions used. Theremaining three compositions containing sodium, potassium, and cesiumdid not gel at the experimental conditions used. Divalent metal ions,such as calcium, magnesium, and zinc, or trivalent metals, such asaluminum or other transition metal ions, are other potential gellingsalts.

EXAMPLE 16 Effect of Iodine and Lithium Concentration on Ionic LiquidElectrolyte Gels

[0091] In this illustrative example, gels were prepared by addinglithium salts to an electrolyte composition containing MeBuImI, iodine,and 2 wt % P4VP. The photovoltaic characteristics of the gels weretested using high-temperature sintered, N3 dye sensitized titanium-oxidephotoelectrodes and platinized SnO₂:F coated glass slides. Both LiI andLiCl gelled the ionic liquid-based compositions that contained smallamounts (2% was sufficient) of complexable polymers like P4VP. Incompositions lacking methylbenzimidazole, the lithium did not effect thephotovoltage. 5 wt % corresponds to a composition including about 376μmoles of lithium salt per gram of ionic liquid and a mixture of 99 wt %butylmethylimidazolium iodide, 1 wt % water, 0.3 M methyl benzimidazole,and 0.25 M iodine. Therefore, 1 wt % corresponds to a 376/5≅75 μmoles oflithium salt per gram of ionic liquid composition. The photovoltaiccharacteristics are summarized in Table 10. TABLE 10 5% LiI 2.5% LiI 5%LiCl 2.5% LiCl 0.05 M Iodine η =  1.6% η =  1.23% η =  0.64% η =  1.19%V_(oc) =  0.6 V V_(oc) =  0.59 V V_(oc) =  0.59 V V_(oc) =  0.58 VI_(sc) =  4.89 mA I_(sc) =  4.21 mA I_(sc) =  2.95 mA I_(sc) =  3.87 mAFF =  0.54 FF =  0.495 FF =  0.36 FF =  0.53 V_(m) =  0.445 V V_(m) = 0.415 V V_(m) =  0.4 V V_(m) =  0.426 V  0.1 M Iodine η =  1.22% η = 1.29% η =  2.83% η =  2.06% V_(oc) =  0.48 V V_(oc) =  0.56 V V_(oc) = 0.57 V_(oc) =  0.58 I_(sc) =  6.46 mA I_(sc) =  5.12 mA I_(sc) =  9.04mA I_(sc) =  7.14 mA FF =  0.39 FF =  0.45 FF =  0.55 FF =  0.5 V_(m) = 0.349 V V_(m) =  0.386 V V_(m) =  0.422 V V_(m) =  0.42 V 0.25 M Iodineη =  2.58% η =  3.06% η =  3.4% η =  2.6% V_(oc) =  0.55 V V_(oc) = 0.55 V V_(oc) =  0.56 V V_(oc) =  0.56 V I_(sc) = 11.49 mA I_(sc) =10.78 mA I_(sc) = 11.32 mA I_(sc) = 10.18 mA FF =  0.41 FF =  0.52 FF = 0.54 FF =  0.46 V_(m) =  0.338 V V_(m) =  0.36 V V_(m) =  0.369 V V_(m)=  0.364 V

EXAMPLE 17 Effect of Polymer Concentration on Gelability andPhotovoltaic Characteristics of Redox Electrolyte Gels

[0092] In this illustrative example, polymer concentration was varied tostudy its effect on gel viscosity and photovoltaic characteristics. Theelectrolyte composition used for this study was a mixture containing 99%MeBuImI, 1% water, 0.25 M iodine, 0.6 M LiI, and 0.3 Mmethylbenzimidazole. The concentration of the polymer, P4VP was variedfrom 1% to 5%. The electrolyte composition with 1% P4VP did flow slowlywhen the vial containing the gel was tilted down. The gels with 2%, 3%,and 5% did not flow. The gel with 5% P4VP appeared much more solid whencompared to the 2% P4VP preparation. Table 11 summarizes thephotovoltaic characteristics of the gels containing the various P4VPcontents that were studied.

[0093] The results show that the photovoltaic characteristics do notvary with the increases in viscosity achieved by increasing the P4VPcontent. Therefore, the viscosity of the gel can be adjusted withoutcausing degradation to the photovoltaic characteristics.Methylbenzimidazole may be necessary to achieve high η. Increasing theiodine concentration up to 0.25 M also increased the efficiency. Beyond0.25 M, the photovoltage decreased drastically, reducing the overallefficiency. Other metal ions or cations like cesium, sodium, potassiumor tetraalkylammonium ions were not found to contribute to theefficiency enhancement and did not cause gelling of the electrolytesolutions. Furthermore, chloride anion was found to enhance theefficiency along with lithium, by improving the photovoltage withoutcausing decreased photoconductivity in compositions containingmethylbenzimidazole. TABLE 11 Photovoltaic Characteristics 1% P4VP 2%P4VP 3% P4VP 5% P4VP η (%) 3.23 3.48 3.09 3.19 I_(sc) (mA/cm²) 10.7410.42 12.03 10.9 V_(oc) (V) 0.59 0.59 0.6 0.61 V_(m) (V) 0.39 0.4 0.380.40 I_(m) (mA/cm²) 8.27 8.69 8.07 8.03 FF 0.51 0.57 0.43 0.48

[0094] C. Co-Sensitizers

[0095] According to one illustrative embodiment, the photosensitizingagent described above includes a first sensitizing dye and secondelectron donor species, the “co-sensitizer.” The first sensitizing dyeand the co-sensitizer may be added together or separately to form thephotosensitized interconnected nanoparticle material 603 shown in FIG.6. As mentioned above with respect to FIG. 6, the sensitizing dyefacilitates conversion of incident visible light into electricity toproduce the desired photovoltaic effect. In one illustrative embodiment,the co-sensitizer donates electrons to an acceptor to form stable cationradicals, which improves the efficiency of charge transfer from thesensitizing dye to the semiconductor oxide nanoparticle material andreduces back electron transfer to the sensitizing dye or co-sensitizer.The co-sensitizer preferably includes (1) conjugation of the freeelectron pair on a nitrogen atom with the hybridized orbitals of thearomatic rings to which the nitrogen atom is bonded and, subsequent toelectron transfer, the resulting resonance stabilization of the cationradicals by these hybridized orbitals; and (2) a coordinating group,such as a carboxy or a phosphate, the function of which is to anchor theco-sensitizer to the semiconductor oxide. Examples of suitableco-sensitizers include, but are not limited to, aromatic amines (e.g.,such as triphenylamine and its derivatives), carbazoles, and otherfused-ring analogues.

[0096] Once again referring back to FIG. 6, the co-sensitizer iselectronically coupled to the conduction band of the photosensitizedinterconnected nanoparticle material 603. Suitable coordinating groupsinclude, but are not limited to, carboxylate groups, phosphates groups,or chelating groups, such as, for example, oximes or alpha ketoenolates.

[0097] Tables 12-18 below present results showing the increase inphotovoltaic cell efficiency when co-sensitizers are co-adsorbed alongwith sensitizing dyes on the surface of high temperature sintered or lowtemperature interconnected titania. In Tables 12-18, characterizationwas conducted using AM 1.5 solar simulator conditions (i.e., irradiationwith light having an intensity of 1000 W/m²). A liquid electrolyteincluding 1 M LiI, 1 M t-butylpyridine, 0.5 M I₂ in3-methoxypropanitrile was employed. The data shown in the tablesindicates an enhancement of one or more operating cell parameters forboth low-temperature-interconnected (Tables 15, 17 and 18) andhigh-temperature-sintered (Tables 12, 13, 14 and 16) titaniananoparticles. The solar cells characteristics listed include η, V_(oc),I_(sc), FF, V_(m), and I_(m). The ratios of sensitizer to co-sensitizerare based on the concentrations of photosensitizing agents in thesensitizing solution.

[0098] In particular, it was discovered that aromatic amines enhancecell performance of dye sensitized titania solar cells if theconcentration of the co-sensitizer is below about 50 mol % of the dyeconcentration. An example of the general molecular structure of thepreferred aromatic amines is shown in FIGS. 13 and 14. Preferably, theconcentration of the co-sensitizer is in the range of about 1 mol % toabout 20 mol %, and more preferably in the range of about 1 mol % toabout 5 mol %.

[0099]FIG. 13A depicts a chemical structure 1300 that may serve as aco-sensitizer. The molecule 1300 adsorbs to the surface of ananoparticle layer via its coordinating group or chelating group, A. Amay be a carboxylic acid group or derivative thereof, a phosphate group,an oxime or an alpha ketoenolate, as described above. FIG. 13B depicts aspecific embodiment 1310 of the structure 1300, namely DPABA(diphenylaminobenzoic acid), where A=COOH. FIG. 13C depicts anotherspecific amine 1320 referred to as DEAPA(N′,N-diphenylaminophenylpropionic acid), with A as the carboxyderivative COOH.

[0100]FIG. 14A shows a chemical structure 1330 that may serve as eithera co-sensitizer, or a sensitizing dye. The molecule does not absorbradiation above 500 nm, and adsorbs to a surface of the nanoparticlelayer via its coordinating or chelating groups, A. A may be a carboxylicacid group or derivative thereof, a phosphate group, an oxime or analpha ketoenolate. R₁ and R₂ may each be a phenyl, alkyl, substitutedphenyl, or benzyl group. Preferably, the alkyl may contain between 1 and10 carbons. FIG. 14B depicts a specific embodiment 1340 of the structure1330, namely DPACA (2,6 bis (4-benzoicacid)-4-(4-N,N-diphenylamino)phenylpyridine carboxylic acid), where R₁ and R₂ are phenyl and A isCOOH.

[0101] DPACA 1340 may be synthesized as follows. 1.49 g (9.08 mmol) of4-acetylbenzoic acid, 1.69 g (6.18 mmol) of 4-N,N-diphenylbenzaldehyde,and 5.8 g (75.2 mmol) of ammonium acetate were added to 60 ml of aceticacid in a 100 ml round bottom flask equipped with a condenser andstirring bar. The solution was heated to reflux with stirring undernitrogen for 5 hours. The reaction was cooled to room temperature andpoured into 150 ml of water, which was extracted with 150 ml ofdichloromethane. The dichloromethane was separated and evaporated with arotary evaporator, resulting in a yellow oil. The oil was then eluted ona silica gel column with 4% methanol/dichloromethane to give theproduct, an orange solid. The solid was washed with methanol and vacuumdried to give 0.920 g of 2,6 bis(4-benzoicacid)-4-(4-N,N-diphenylamino)phenylpyridine (DPACA). Themelting point was 199°-200° C., the λ_(max) was 421 nm, and the molarextinction coefficient, E was 39,200 L mole⁻¹ cm⁻¹. The structure wasconfirmed by NMR spectroscopy.

[0102] Table 12 shows the results for high-temperature-sintered titania;photosensitized by overnight soaking in solutions of 1 mM N3 dye andthree concentrations of DPABA. Table 12 also shows that the average η isgreatest for the preferred 20/1 (dye/co-sensitizer) ratio. TABLE 12 I-VCHARACTERIZATION Cell General area V_(oc) I_(m) V_(m) I_(sc) ηconditions Conditions cm² V mA/cm² V mA/cm² FF % σ Adsorption 1 mM 0.440.62 6.69 0.44 8.38 0.56 2.91 Temp. N3/EtOH, 0.52 0.64 6.81 0.43 8.590.54 2.94 RT ° C. Overnight 0.54 0.63 6.95 0.41 8.72 0.52 2.84 CONTROLEtOH Average 0.50 0.63 6.82 0.43 8.56 0.54 2.90 0.05 Dye Concen. 1 mMN3, 0.50 0.64 7.70 0.45 9.31 0.58 3.43 0.05 mM N3, DPABA DPABA in 0.530.64 7.40 0.45 9.30 0.56 3.31 EtOH for 0.50 0.64 7.70 0.45 9.38 0.573.44 Overnight; 20/1 Sintering Average 0.51 0.64 7.60 0.45 9.33 0.573.39 0.07 Temp 450° C., 30 minutes Thickness of 1 mM N3, 1 0.53 0.637.21 0.41 8.58 0.55 2.96 Film mM DPABA 0.50 0.63 6.75 0.44 8.23 0.572.97 TiO₂, ˜10 μm in EtOH for 0.42 0.63 7.11 0.44 8.67 0.57 3.13Overnight; 1/1 Average 0.48 0.63 7.02 0.43 8.49 0.56 3.02 0.10Electrolyte 1 mM N3, 10 0.33 0.58 4.95 0.42 6.02 0.60 2.08 AM 1.5D, l 1mM DPABA 0.52 0.60 5.51 0.42 6.67 0.58 2.31 Sun in EtOH for 0.49 0.605.53 0.42 6.72 0.58 2.32 Overnight; 1/10 Film Average 0.45 0.59 5.330.42 6.47 0.58 2.24 0.14 pretreatment

[0103] Table 13 shows the results of using a cut-off filter (third andfourth entries) while irradiating a cell to test its I-Vcharacteristics. Table 13 also shows that the efficiency of the cellstill improves when DPABA is present, indicating that its effect when nofilter is present is not simply due to absorption of UV light by DPABAfollowed by charge injection. FIG. 14 shows a plot 1400 of theabsorbance versus wavelength for the cut-off filter used to characterizethe photovoltaic cells, according to an illustrative embodiment of theinvention. FIG. 15 shows a plot 1500 of the absorbance versus wavelengthfor DPABA, which absorbs below 400 nm. Because the absorbance of thecut-off is large, little light reaches the absorption bands of DPABA.TABLE 13 I-V CHARACTERIZATION Cell area V_(oc) I_(m) V_(m) I_(sc) ηConditions cm² V mA/cm² V mA/cm² FF % σ 1 mM N3 in 0.49 0.70 8.62 0.4611.02 0.51 3.97 EtOH Overnight 0.49 0.70 8.13 0.45 10.20 0.51 3.66control 0.49 0.73 7.93 0.51 9.69 0.57 4.04 Average 0.49 0.71 8.23 0.4710.30 0.53 3.89 0.20 1 mM N3 0.49 0.71 9.05 0.46 11.53 0.51 4.16 0.05 mMDPABA in 0.49 0.71 9.24 0.46 11.56 0.52 4.25 EtOH, 20/1 Overnight 0.490.71 9.39 0.46 11.50 0.53 4.32 Average 0.49 0.71 9.23 0.46 11.53 0.524.24 0.08 1 mM N3 in 0.49 0.69 6.35 0.47 7.83 0.55 4.26 455 nm cut EtOHoff filter Overnight 0.49 0.69 6.05 0.46 7.44 0.54 3.98 used, control 700.49 0.72 5.74 0.52 6.94 0.60 4.27 mW/cm² Average 0.49 0.70 6.05 0.487.40 0.56 4.17 0.17 1 mM N3 0.49 0.70 6.73 0.47 8.21 0.55 4.52 455 nmcut 0.05 mM off filter DPABA in 0.49 0.70 6.74 0.47 8.19 0.55 4.56 used,EtOH, 20/1 70 Overnight 0.49 0.70 6.74 0.49 8.25 0.57 4.72 mW/cm²Average 0.49 0.70 6.74 0.48 8.22 0.56 4.59 0.11

[0104] Table 14 shows that the addition of triphenylamine itself (i.e.,no titania complexing groups such as carboxy) does not significantlyenhance efficiency under the stated conditions. TABLE 14 I-VCHARACTERIZATION Cell area V_(oc) I_(m) V_(m) I_(sc) η Conditions cm² VmA/cm² V mA/cm² FF % σ 0.5 mM N3 0.49 0.70 7.96 0.45 9.82 0.52 3.58 inEtOH, Overnight 0.49 0.71 8.09 0.48 9.58 0.57 3.88 0.49 0.70 7.47 0.488.83 0.58 3.59 Average 0.49 0.70 7.84 0.47 9.41 0.56 3.68 0.17 0.5 mMN3, 0.49 0.69 7.44 0.45 9.21 0.53 3.35 0.025 mM TPA in 0.49 0.69 7.610.47 9.75 0.53 3.58 EtOH Overnight 0.49 0.69 6.98 0.45 8.56 0.53 3.1420/1 Average 0.49 0.69 7.34 0.46 9.17 0.53 3.36 0.22 0.5 mM N3, 0.490.68 4.62 0.44 5.66 0.53 2.03 2.0 mM TPA in 0.49 0.66 4.18 0.45 5.380.53 1.88 EtOH Overnight 0.49 0.66 4.51 0.45 5.82 0.53 2.03 ¼ Average0.49 0.67 4.44 0.45 5.62 0.53 1.98 0.09

[0105] Table 15 shows that the effect is present using low temperatureinterconnected titania and that the 20/1 (dye/co-sensitizer) ratio ispreferred. TABLE 15 I-V CHARACTERIZATION Cell area V_(oc) I_(m) V_(m)I_(sc) η Conditions cm² V mA/cm² V mA/cm² FF % σ 0.5 mM 0.49 0.73 8.320.50 10.56 0.54 4.16 N3/EtOH, overnight, 0.51 0.72 8.13 0.49 10.30 0.543.98 control 0.50 0.72 8.56 0.47 10.65 0.52 4.02 Average 0.50 0.72 8.340.49 10.50 0.53 4.06 0.09 0.5 mM N3, 0.49 0.73 8.55 0.51 10.48 0.57 4.360.0125 mM DPABA in 0.53 0.72 8.53 0.50 11.00 0.54 4.27 EtOH, 40/1,overnight 0.49 0.74 8.08 0.54 10.96 0.54 4.36 Average 0.50 0.73 8.390.52 10.81 0.55 4.33 0.06 0.5 mM N3, 0.49 0.73 9.07 0.49 11.31 0.54 4.440.017 mM DPABA in 0.49 0.75 8.64 0.52 10.97 0.55 4.49 EtOH, 30/1,overnight 0.52 0.73 8.19 0.52 10.88 0.54 4.26 Average 0.50 0.74 8.630.51 11.05 0.54 4.40 0.12 0.5 mM N3, 0.50 0.75 8.57 0.52 11.56 0.51 4.460.025 mM DPABA in 0.49 0.74 8.88 0.52 11.45 0.54 4.62 EtOH, 20/1,overnight 0.53 0.74 9.01 0.51 12.08 0.51 4.60 Average 0.51 0.74 8.820.52 11.70 0.52 4.56 0.09 0.5 mM N3, 0.49 0.72 8.85 0.48 10.78 0.55 4.250.5 mM DPABA in 0.51 0.74 8.62 0.47 10.37 0.53 4.05 EtOH, 1/1, overnight0.50 0.75 8.38 0.49 10.02 0.55 4.11 Average 0.50 0.74 8.62 0.48 10.390.54 4.14 0.10 0.5 mM N3, 0.49 0.68 7.56 0.44 9.09 0.54 3.33 5 mM DPABAin 0.51 0.69 7.62 0.46 9.34 0.54 3.51 EtOH, 1/10, overnight 0.49 0.677.25 0.45 8.84 0.55 3.26 Average 0.50 0.68 7.48 0.45 9.09 0.54 3.36 0.13

[0106] Table 16 shows results for high-temperature-sintered titaniasensitized with a high concentration of N3 dye while maintaining a 20/1ratio of dye to co-sensitizer. Entries 1 and 2 show the increase in cellperformance due to co-sensitizer. Entry 3 shows the effect of DPABAalone as a sensitizer, demonstrating that this material acts as asensitizer by itself when irradiated with the full solar spectrum, whichincludes low-intensity UV radiation. TABLE 16 I-V CHARACTERIZATIONGeneral Cell area V_(oc) I_(m) V_(m) I_(sc) η Conditions Conditions cm²V mA/cm² V mA/cm² FF % σ Adsorption 8 mM 0.49 0.68 8.51 0.44 10.07 0.553.74 Temp. N3/aprotic 0.49 0.67 8.28 0.44 9.75 0.56 3.64 RT ° C. polarsolvent, Solvent of Dye 1 hour 0.49 0.68 9.16 0.42 10.80 0.52 3.85CONTROL Aprotic polar average 0.49 0.68 8.65 0.43 10.21 0.54 3.74 0.10solvent 8 mM N3, 0.4 0.49 0.68 9.52 0.44 11.18 0.55 4.19 mM DPABA inaprotic polar 0.49 0.68 9.96 0.44 11.59 0.56 4.38 solvent, 20/1 0.490.65 9.81 0.42 12.13 0.52 4.12 1 hour average 0.49 0.67 9.76 0.43 11.630.54 4.23 0.14 5 mM DPABA 0.49 0.55 1.02 0.42 1.22 0.64 0.43 in aproticpolar 0.49 0.55 0.94 0.41 1.13 0.62 0.39 solvent Overnight 0.49 0.580.89 0.44 1.07 0.63 0.39 0.49 0.56 0.95 0.42 1.14 0.63 0.40 0.02

[0107] Table 17 shows results for low-temperature-interconnectedtitania. Entry 5 shows the affect of DPACA alone as a sensitizer,demonstrating that this material acts as a sensitizer by itself whenirradiated with the full solar spectrum, which includes low-intensity UVradiation. TABLE 17 I-V CHARACTERIZATION Cell area V_(oc) I_(m) V_(m)I_(sc) η Conditions cm² V mA/cm² V mA/cm² FF % σ 0.5 mM 0.51 0.73 8.400.50 10.84 0.53 4.20 N3/EtOH, overnight, 0.53 0.72 8.13 0.49 10.30 0.543.98 control 0.50 0.72 8.77 0.47 10.87 0.53 4.12 average 0.51 0.72 8.430.49 10.67 0.53 4.10 0.11 0.5 mM N3, 0.49 0.73 8.10 0.51 10.39 0.54 4.130.01 mM DPACA in 0.50 0.74 7.95 0.50 10.01 0.54 3.98 EtOH, 50/1,overnight 0.49 0.72 8.10 0.50 9.85 0.57 4.05 average 0.49 0.73 8.05 0.5010.08 0.55 4.05 0.08 0.5 mM N3, 0.49 0.74 8.38 0.50 10.48 0.54 4.19 0.02mM DPACA in 0.52 0.73 8.18 0.48 9.74 0.55 3.93 EtOH, 25/1, overnight0.49 0.76 8.08 0.54 9.45 0.61 4.36 average 0.50 0.74 8.21 0.51 9.89 0.574.16 0.22 0.5 mM N3, 0.49 0.73 9.07 0.46 11.31 0.51 4.17 0.5 mM DPACA in0.49 0.75 7.41 0.53 9.24 0.57 3.93 EtOH, 1/1, overnight 0.52 0.76 7.930.52 9.12 0.59 4.12 average 0.50 0.75 8.14 0.50 9.89 0.56 4.07 0.13 0.5mM N3, 0.56 0.73 6.36 0.49 7.59 0.56 3.12 5.0 mM DPACA in 0.52 0.73 6.630.49 7.84 0.57 3.25 EtOH, 1/10, overnight 0.50 0.72 6.53 0.49 7.59 0.593.20 average 0.53 0.73 6.51 0.49 7.67 0.57 3.19 0.07 5.0 mM 0.43 0.653.12 0.49 3.77 0.62 1.53 DPACA in 0.45 0.65 2.93 0.49 3.51 0.63 1.44EtOH, overnight 0.49 0.66 2.83 0.49 3.40 0.62 1.39 average 0.46 0.652.96 0.49 3.56 0.62 1.45 0.07

[0108] Table 18 shows results for low-temperature-interconnectedtitania. Entry 6 shows the affect of DEAPA alone as a sensitizer,demonstrating that this material acts as a sensitizer by itself whenirradiated with the full solar spectrum, which includes low-intensity UVradiation. TABLE 18 I-V CHARACTERIZATION General Cell area V_(oc) I_(m)V_(m) I_(sc) η conditions Conditions cm² V mA/cm² V mA/cm² FF % σAdsorption 0.5 mM 0.51 0.72 8.67 0.49 10.60 0.56 4.25 Temp. N3/EtOH, RT° C. overnight, 0.49 0.75 8.15 0.47 10.50 0.49 3.83 control 0.49 0.748.74 0.44 10.63 0.49 3.85 Solvent of average 0.50 0.74 8.52 0.47 10.580.51 3.97 0.24 Dye EtOH Dye Concen. 0.5 mM N3, 0.49 0.70 8.68 0.44 11.000.50 3.82 0.01 mM N3, DEAPA DEAPA in 0.52 0.71 8.57 0.45 11.11 0.49 3.86EtOH, 50/1, overnight 0.50 0.72 8.40 0.45 10.61 0.49 3.78 Sinteringaverage 0.50 0.71 8.55 0.45 10.91 0.49 3.82 0.04 Temp 120° C., 10minutes Thickness of 0.5 mM N3, 0.51 0.74 8.90 0.44 10.92 0.48 3.92 Film0.02 mM TiO₂, ˜7 μm DEAPA in 0.53 0.73 8.76 0.44 10.51 0.50 3.85 EtOH,25/1, overnight 0.49 0.73 8.40 0.45 10.21 0.51 3.78 Liquid average 0.510.73 8.69 0.44 10.55 0.50 3.85 0.07 Electrolyte 0.5 mM N3, 0.49 0.718.94 0.43 10.78 0.50 3.84 0.5 mM AM 1.5D, 1 DEAPA in 0.51 0.71 8.83 0.4410.37 0.53 3.89 Sun EtOH, 1/1, overnight 0.50 0.70 8.18 0.42 9.71 0.513.44 Film average 0.50 0.71 8.65 0.43 10.29 0.51 3.72 0.25 pretreatment0.5 mM N3, 0.52 0.60 0.88 0.45 1.08 0.61 0.40 5.0 mM DEAPA in 0.49 0.590.71 0.44 0.85 0.62 0.31 EtOH, 1/10, overnight 0.49 0.59 0.75 0.44 0.910.61 0.33 average 0.50 0.59 0.78 0.44 0.95 0.62 0.35 0.04 5.0 mM 0.490.54 0.41 0.42 0.49 0.65 0.17 DEAPA in 0.49 0.54 0.35 0.39 0.46 0.550.14 CHCl3, overnight 0.51 0.52 0.45 0.40 0.52 0.67 0.18 average 0.500.53 0.40 0.40 0.49 0.62 0.16 0.02

[0109] D. Semiconductor Oxide Formulations

[0110] In a further illustrative embodiment, the invention providessemiconductor oxide formulations for use with DSSCs formed using a lowtemperature semiconductor oxide nanoparticle interconnection, asdescribed above. The semiconductor oxide formulations may be coated atroom temperature and, upon drying at temperatures between about 50° C.and about 150° C., yield mechanically stable semiconductor nanoparticlefilms with good adhesion to the transparent conducting oxide (TCO)coated plastic substrates. In one embodiment, the nanoparticlesemiconductor of the photosensitized interconnected nanoparticlematerial 603 is formed from a dispersion of commercially available TiO₂nanoparticles in water, a polymer binder, with or without acetic acid.The polymer binders used include, but are not limited to,polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), hydroxyethylcellulose (HOEC), hydroxypropyl cellulose, polyvinyl alcohol (PVA) andother water-soluble polymers. The ratio of semiconductor oxideparticles, e.g., TiO₂, to polymer can be between about 100:0.1 to 100:20by weight, and preferably is between about 100:1 to 100:10 by weight.The presence of acetic acid in the formulation helps to improve theadhesion of the coating to the TCO coated substrate. However, aceticacid is not essential to this aspect of the invention and semiconductoroxide dispersions without acetic acid perform satisfactorily. In anotherembodiment, the TiO₂ nanoparticles are dispersed in an organic solvent,such as, e.g., isopropyl alcohol, with polymeric binders such as, e.g.,PVP, butvar, ethylcellulose, etc.

[0111] In another illustrative embodiment, the mechanical integrity ofthe semiconductor oxide coatings and the photovoltaic performance of thedye sensitized cells based on these coatings can be further improved byusing a crosslinking agent to interconnect the semiconductornanoparticles. The polylinkers described above may be used for thispurpose. These crosslinking agents can be applied, e.g., in the titaniacoating formulation directly or in a step subsequent to drying thetitania coating as a solution in an organic solvent such as ethanol,isopropanol or butanol. For example, subsequent heating of the films totemperatures in the range of about 70° C. to about 140° C. leads to theformation of TiO₂ bridges between the TiO₂ nanoparticles. Preferably,the concentration of the polylinker in this example ranges from about0.01 to about 20 weight % based on titania.

[0112] E. Semiconductor Primer Layer Coatings

[0113] In another illustrative embodiment, the invention providessemiconductor oxide materials and methods of coating semiconductor oxidenanoparticle layers on a base material to form DSSCs. FIG. 16 depicts anillustrative embodiment 1600 of the coating process, according to theinvention. In this illustrative embodiment, a base material 1610 iscoated with a first primer layer 1620 of a semiconductor oxide, and thena suspension of nanoparticles 1630 of the semiconductor oxide is coatedover the primer layer 1620. The primer layer 1620 may include avacuum-coated semiconductor oxide film (e.g., a TiO₂ film).Alternatively, the primer layer 1620 may include a thin coating withfine particles of a semiconductor oxide (e.g. TiO₂, SnO₂). The primerlayer 1620 may also include a thin layer of a polylinker or precursorsolution, one example of which is the Ti (IV) butoxide polymer 400 shownin FIG. 4 above. According to one illustrative embodiment of theinvention, the base material 1610 is the first flexible, significantlylight transmitting substrate 609 referred to in FIG. 6. Additionally,the base material 1610 is a transparent, conducting, plastic substrate.According to this illustrative embodiment, the suspension ofnanoparticles 1630 is the photosensitized interconnected nanoparticlematerial 603 of FIG. 6. Numerous semiconducting metal oxides, includingSnO₂, TiO₂, Ta₂O₅, Nb₂O₅, and ZnO, among others, in the form of thinfilms, fine particles, or precursor solutions may be used as primerlayer coatings using vacuum coating, spin coating, blade coating orother coating methods.

[0114] The primer layer 1620 improves the adhesion of nano-structuredsemiconductor oxide films, like layer 1630, to the base material 1610.Enhancements in the performance of DSSCs with such primer layers havebeen observed and will be described below. The enhancement arises froman increase in the adhesion between the semiconductor oxidenanoparticles (or photoelectrodes) and the transparent conducting oxidecoated plastic substrates, as well as from higher shunt resistance.

[0115] Examples of various illustrative embodiments of this aspect ofthe invention, in the context of a DSSC including a titanium dioxidenanoparticle layer, are as follows.

EXAMPLE 18 Vacuum Coated TiO₂ as Prime Layers for Nanoparticle TiO₂Photoelectrodes

[0116] In this illustrative example, thin TiO₂ films with thicknessesranging from 2.5 nm to 100 nm were sputter-coated under vacuum on an ITOlayer coated on a polyester (here, PET) substrate. A water based TiO₂(P25, with an average particle size of 21 nm) slurry was spin-coated onboth the ITO/PET with sputter-coated thin TiO₂ and on the plain ITO/PET(i.e., the portion without sputter-coated thin TiO₂). The coated filmswere soaked in poly [Ti(OBu)₄] solution in butanol and then heat treatedat 120° C. for 2 minutes. The low-temperature reactively interconnectedfilms were placed into an aprotic, polar solvent-based N3 dye solution(8 mM) for 2 minutes. Photovoltaic cells were made with platinum (Pt)counter-electrodes, an I⁻/I₃ ⁻ liquid electrolyte, 2 mil SURLYN, andcopper conducting tapes. I-V characterization measurements wereperformed with a solar simulator.

[0117] Adhesion of nanostructured TiO₂ films from the P25 slurry coatedon the ITO/PET with sputter-coated, thin TiO₂ was superior to films onthe plain ITO/PET. Better photovoltaic performance was also observedfrom the PV cells prepared on the ITO/PET with sputter-coated, thin TiO₂as compared to those on the plain ITO/PET. Improvement on thefill-factor was achieved as well. A FF as high as 0.67 was measured forthe photovoltaic cells made on the ITO/PETs with sputter-coated, thinTiO₂. For the photovoltaic cells made on the plain ITO/PET, the FFobserved was not greater than 0.60. Higher photovoltaic conversionefficiencies (about 17% higher than the photoelectrodes made from theplain ITO/PET) were measured for the photoelectrodes prepared on theITO/PET with thin sputter-coated TiO₂. Improvement in shunt resistancewas also observed for the photovoltaic cells made on the ITO/PET withthin sputter-coated TiO₂.

EXAMPLE 19 Fine Particles of TiO₂ as Primer Layer for TiO₂ Suspensions

[0118] In this illustrative example, fine particles of TiO₂, smallenough such that they would stick in the valleys between spikes of ITOon the PET substrate, were prepared by hydrolyzing titanium (IV)isopropoxide. The fine particles were then spin coated at 800 rpm ontothe ITO layer. A 37% TiO₂ (P25) suspension of approximately 21 nmaverage particle size was then spin coated at 800 rpm onto the fineparticle layer. The coated TiO₂ was low temperature interconnected bydipping in 0.01 molar Ti (IV) butoxide polymer in butanol for 15 minutesfollowed drying on a slide warmer at 50° C. before heating at 120° C.for 2 minutes. The interconnected coating was dyed with N3 dye bydipping into an 8 mM aprotic polar solvent solution for 2 minutes, thenrinsed with ethanol and dried on a slide warmer at 50° C. for 2 minutes.Control coatings were prepared in the same way, except without the fineparticle prime coat. The cells' performance characteristics weremeasured using a solar simulator. Results for test and control arelisted below in Table 19. Fine particles of tin oxide as primer coatingfor TiO₂ suspensions yielded similar improvements. TABLE 19 V_(oc)I_(sc) η FF Control 0.64 4.86 1.67% 0.54 Invention 0.66 6.27 2.36% 0.57

EXAMPLE 20 Titanium (IV) Butoxide Polymer in Butanol (PrecursorSolution) as Primer Layer for TiO₂

[0119] In another test, titanium (IV) butoxide polymer in butanol at0.01 molar was spin coated on an ITO/PET plastic base at 800 rpm. A 43%TiO₂ (P25) suspension of approximately 21 nm average particle size wasspin coated at 800 rpm. The coated TiO₂ was interconnected at lowtemperature by dipping in 0.01 M titanium (IV) butoxide polymer inbutanol for 15 minutes and then drying on a slide warmer at 50° C.before heating at 120° C. for 2 minutes. The sintered coating was dyedwith N3 dye by dipping into an 8 mM aprotic, polar solvent solution for2 minutes, then rinsed with ethanol and dried on a slide warmer at 50°C. for 2 minutes. Control coatings were prepared in the same way onlywithout the primer layer coating. The I-V properties of the cells weremeasured with a solar simulator. Results for test and control are listedbelow in Table 20. TABLE 20 V_(oc) I_(sc) η FF Control 0.66 7.17 2.62%0.56 Invention 0.70 8.11 3.38% 0.59

[0120] While the invention has been particularly shown and describedwith reference to specific illustrative embodiments, it should beunderstood that various changes in form and detail may be made withoutdeparting from the spirit and scope of the invention as defined by theappended claims. By way of example, any of the disclosed features may becombined with any of the other disclosed features to form a photovoltaiccell or module.

What is claimed is:
 1. A method of interconnecting nanoparticles at lowtemperature, the method comprising the steps of: providing a solutioncomprising a polymeric linking agent and a solvent; and contacting aplurality of metal oxide nanoparticles with the solution at atemperature below about 300° C. to interconnect at least a portion ofthe plurality of metal oxide nanoparticles.
 2. The method of claim 1,wherein the temperature is below about 200° C.
 3. The method of claim 1,wherein the temperature is below about 100° C.
 4. The method of claim 1,wherein the temperature is about room temperature.
 5. The method ofclaim 1, wherein the polymeric linking agent comprises a long chainmacromolecule.
 6. The method of claim 5, wherein the long chainmacromolecule comprises: a backbone structure substantially similar to achemical structure of the plurality of metal oxide nanoparticles; andone or more reactive groups chemically bonded to the backbone structure.7. The method of claim 1, wherein the plurality of metal oxidenanoparticles comprise a chemical structure, M_(x)O_(y), wherein x and yare integers.
 8. The method of claim 7, wherein M comprises one of thegroup comprising Ti, Zr, Sn, W, Nb, Ta, and Tb.
 9. The method of claim1, wherein the polymeric linking agent comprises poly(n-butyl titanate).10. The method of claim 1, wherein the solvent of the solution comprisesn-butanol.
 11. The method of claim 6, wherein at least a portion of theplurality of metal oxide nanoparticles are interconnected via amechanical bridge formed by the one or more reactive groups binding withthe plurality of metal oxide nanoparticles.
 12. The method of claim 6,wherein at least a portion of the plurality of metal oxide nanoparticlesare interconnected via an electrical bridge formed by the one or morereactive groups binding with the plurality of metal oxide nanoparticles.13. The method of claim 1, wherein the plurality of metal oxidenanoparticles are disposed as a thin film on a substrate.
 14. The methodof claim 13, wherein the substrate is dipped into the solutioncomprising the polymeric linking agent.
 15. The method of claim 13,wherein the solution comprising the polymeric linking agent is sprayedonto the substrate.
 16. The method of claim 1, wherein the solutioncomprising the polymeric linking agent is dispersed on a substrate. 17.The method of claim 16, wherein the plurality of metal oxidenanoparticles are deposited on the substrate comprising the solutioncomprising the polymeric linking agent.
 18. The method of claim 1,further comprising the step of contacting the metal oxide nanoparticleswith a modifier solution.
 19. The method of claim 1, wherein theplurality of metal oxide nanoparticles comprises nanoparticles ofmaterials selected from the group consisting of titanium oxides,zirconium oxides, zinc oxides, tungsten oxides, niobium oxides,lanthanum oxides, tin oxides, terbium oxides, tantalum oxides, and oneor more combinations thereof.
 20. A polymeric linking agent solutioncomprising: a polymeric linking agent of the formula—[O-M(OR)_(i)—]_(m)—; a plurality of metal oxide nanoparticles comprisedof the formula M_(x)O_(y); and a solvent, wherein (i) i, m, x, and y areintegers greater than zero, (ii) M is selected from the group consistingof Ti, Zr, Sn, W, Nb, Ta, and Tb, (iii) R is from the group consistingof hydrogen, alkyls, alkenes, alkynes, aromatics, and acyls, and (iv)the solution contains the polymeric linking agent in a concentrationsufficient to interconnect at least a portion of the plurality of metaloxide nanoparticles at a temperature below about 300° C.
 21. Thepolymeric linking agent solution of claim 20, wherein the solutioncontains the polymeric linking agent in a concentration sufficient tointerconnect at least a portion of the plurality of nanoparticles at atemperature below about 100° C.
 22. A flexible photovoltaic cellcomprising a photosensitized interconnected nanoparticle material and acharge carrier material, both disposed between first and secondflexible, significantly light transmitting substrates.
 23. Thephotovoltaic cell of claim 22, wherein the photosensitizedinterconnected nanoparticle material comprises nanoparticles linked by apolymeric linking agent.
 24. The photovoltaic cell of claim 22, whereinthe photosensitized interconnected nanoparticle material comprisesparticles with an average size substantially in the range of about 5 nmto about 80 nm.
 25. The photovoltaic cell of claim 22, wherein thephotosensitized interconnected nanoparticle material comprisesinterconnected titanium dioxide nanoparticles.
 26. The photovoltaic cellof claim 22, wherein the photosensitized interconnected nanoparticlematerial is selected from the group consisting of zirconium oxides, zincoxides, tungsten oxides, niobium oxides, lanthanum oxides, tantalumoxides, tin oxides, terbium oxides, and combinations thereof.
 27. Thephotovoltaic cell of claim 22, wherein the photosensitizedinterconnected nanoparticle material comprises a photosensitizing agentthat comprises a molecule selected from the group consisting ofxanthines, cyanines, merocyanines, phthalocyanines, and pyrroles. 28.The photovoltaic cell of claim 27, wherein the photosensitizing agentfurther comprises a metal ion selected from the group consisting ofdivalent and trivalent metals.
 29. The photovoltaic cell of claim 27,wherein the photosensitizing agent comprises at least one of a rutheniumtransition metal complex, an osmium transition metal complex, and aniron transition metal complex.
 30. The photovoltaic cell of claim 22,wherein the charge carrier material comprises a redox electrolytesystem.
 31. The photovoltaic cell of claim 22, wherein the chargecarrier material comprises a polymeric electrolyte.
 32. The photovoltaiccell of claim 22, wherein the charge carrier material transmits at leastabout 60% of incident visible light.
 33. The photovoltaic cell of claim22, wherein at least one of the first and second flexible, significantlylight transmitting substrates comprises a polyethylene terephthalatematerial.
 34. The photovoltaic cell of claim 22, further comprising acatalytic media layer disposed between the flexible, significantly lighttransmitting first and second substrates.
 35. The photovoltaic cell ofclaim 34, wherein the catalytic media layer comprises platinum.
 36. Thephotovoltaic cell of claim 22, further comprising an electricalconductor material disposed on at least one of the first and secondflexible, significantly light transmitting substrates.
 37. Thephotovoltaic cell of claim 36, wherein the electrical conductor materialcomprises indium tin oxide.